Ellis-van Creveld syndrome is an autosomal recessive skeletal dysplasia characterized by short limbs, short ribs, postaxial polydactyly, and dysplastic nails and teeth. Congenital cardiac defects, most commonly a defect of primary atrial septation producing a common atrium, occur in 60% of affected individuals (summary by Ruiz-Perez et al., 2000). The clinical features of the Ellis-van Creveld syndrome appear to be identical regardless of whether the disorder is caused by mutation in the EVC gene (604831) or in the EVC2 gene (607261) (Ruiz-Perez et al., 2003, Galdzicka et al., 2002).

Orofaciodigital syndrome II (OFD2), also known as Mohr syndrome, is characterized by cleft lip/palate, lobulated tongue with nodules, dental anomalies including tooth agenesis, maxillary hypoplasia, conductive hearing loss, and poly-, syn-, and brachydactyly. Mesomelic shortening of the limbs has also been observed (Mohr, 1941; Gorlin, 1982; Monroe et al., 2016).

Smith-Lemli-Opitz syndrome (SLOS) is a congenital multiple-anomaly / cognitive impairment syndrome caused by an abnormality in cholesterol metabolism resulting from deficiency of the enzyme 7-dehydrocholesterol (7-DHC) reductase. It is characterized by prenatal and postnatal growth restriction, microcephaly, moderate-to-severe intellectual disability, and multiple major and minor malformations. The malformations include distinctive facial features, cleft palate, cardiac defects, underdeveloped external genitalia in males, postaxial polydactyly, and 2-3 syndactyly of the toes. The clinical spectrum is wide; individuals with normal development and only minor malformations have been described.

GLI3-related Pallister-Hall syndrome (GLI3-PHS) is characterized by a spectrum of anomalies ranging from polydactyly, asymptomatic bifid epiglottis, and hypothalamic hamartoma at the mild end to laryngotracheal cleft with neonatal lethality at the severe end. Individuals with mild GLI3-PHS may be incorrectly diagnosed as having isolated postaxial polydactyly type A. Individuals with GLI3-PHS can have pituitary insufficiency and may die as neonates from undiagnosed and untreated adrenal insufficiency.

Typical Greig cephalopolysyndactyly syndrome (GCPS) is characterized by macrocephaly, widely spaced eyes associated with increased interpupillary distance, preaxial polydactyly with or without postaxial polydactyly, and cutaneous syndactyly. Developmental delay, intellectual disability, or seizures appear to be uncommon manifestations (~<10%) of GCPS and may be more common in individuals with large (>300-kb) deletions that encompass GLI3. Approximately 20% of individuals with GCPS have hypoplasia or agenesis of the corpus callosum.

Pallister-Killian syndrome (PKS) is a dysmorphic condition involving most organ systems, but is also characterized by a tissue-limited mosaicism; most fibroblasts have 47 chromosomes with an extra small metacentric chromosome, whereas the karyotype of lymphocytes is normal. The extra metacentric chromosome is an isochromosome for part of the short arm of chromosome 12: i(12)(p10) (Peltomaki et al., 1987; Warburton et al., 1987).

Pallister-Hall-like syndrome (PHLS) is a pleiotropic autosomal recessive disorder characterized by phenotypic variability. Patients exhibit postaxial polydactyly as well as hypothalamic hamartoma, cardiac and skeletal anomalies, and craniofacial dysmorphisms. Hirschsprung disease has also been observed (Rubino et al., 2018; Le et al., 2020). Pallister-Hall syndrome (146510) is an autosomal dominant disorder with features overlapping those of PHLS, caused by mutation in the GLI3 gene (165240).

Other features occur in only one or a few types of oral-facial digital syndrome. These features help distinguish the different forms of the disorder. For example, the most common form of oral-facial-digital syndrome, type I, is associated with polycystic kidney disease. This kidney disease is characterized by the growth of fluid-filled sacs (cysts) that interfere with the kidneys' ability to filter waste products from the blood. Other forms of oral-facial-digital syndrome are characterized by neurological problems, particular changes in the structure of the brain, bone abnormalities, vision loss, and heart defects.\n\nDistinctive facial features often associated with oral-facial-digital syndrome include a split in the lip (a cleft lip); a wide nose with a broad, flat nasal bridge; and widely spaced eyes (hypertelorism).\n\nAbnormalities of the oral cavity that occur in many types of oral-facial-digital syndrome include a split (cleft) in the tongue, a tongue with an unusual lobed shape, and the growth of noncancerous tumors or nodules on the tongue. Affected individuals may also have extra, missing, or defective teeth. Another common feature is an opening in the roof of the mouth (a cleft palate). Some people with oral-facial-digital syndrome have bands of extra tissue (called hyperplastic frenula) that abnormally attach the lip to the gums.\n\nAbnormalities of the digits can affect both the fingers and the toes in people with oral-facial-digital syndrome. These abnormalities include fusion of certain fingers or toes (syndactyly), digits that are shorter than usual (brachydactyly), or digits that are unusually curved (clinodactyly). The presence of extra digits (polydactyly) is also seen in most forms of oral-facial-digital syndrome.\n\nThe signs and symptoms of oral-facial-digital syndrome vary widely. However, most forms of this disorder involve problems with development of the oral cavity, facial features, and digits. Most forms are also associated with brain abnormalities and some degree of intellectual disability.\n\nResearchers have identified at least 13 potential forms of oral-facial-digital syndrome. The different types are classified by their patterns of signs and symptoms. However, the features of the various types overlap significantly, and some types are not well defined. The classification system for oral-facial-digital syndrome continues to evolve as researchers find more affected individuals and learn more about this disorder.\n\nOral-facial-digital syndrome is actually a group of related conditions that affect the development of the oral cavity (the mouth and teeth), facial features, and digits (fingers and toes).

Weyers acrofacial dysostosis (WAD) is an autosomal dominant disorder with dental anomalies, nail dystrophy, postaxial polydactyly, and mild short stature. Ellis-van Creveld syndrome is a similar disorder, with autosomal recessive inheritance and the additional features of disproportionate dwarfism, thoracic dysplasia, and congenital heart disease (summary by Howard et al., 1997).

Microphthalmia with limb anomalies (MLA), also known as Waardenburg anophthalmia syndrome or ophthalmoacromelic syndrome (OAS), is a rare autosomal recessive developmental disorder characterized by unilateral or bilateral microphthalmia, clinical anophthalmia, syndactyly, polydactyly, synostosis, or oligodactyly. Long-bone hypoplasia and renal, venous, and vertebral anomalies may also be present. Impaired intellectual development is present in about half of affected individuals (summary by Tekin et al., 2000, Abouzeid et al., 2011).

The C syndrome, also known as Opitz trigonocephaly syndrome, is a malformation syndrome characterized by trigonocephaly, severe mental retardation, hypotonia, variable cardiac defects, redundant skin, and dysmorphic facial features, including upslanted palpebral fissures, epicanthal folds, depressed nasal bridge, and low-set, posteriorly rotated ears (summary by Kaname et al., 2007). C syndrome shows phenotypic overlap with Bohring-Opitz syndrome, or C-like syndrome (605039), a disorder with more severe features than C syndrome, caused by heterozygous mutation in the ASXL1 gene (612990) on chromosome 20q11.

Classic Joubert syndrome (JS) is characterized by three primary findings: A distinctive cerebellar and brain stem malformation called the molar tooth sign (MTS). Hypotonia. Developmental delays. Often these findings are accompanied by episodic tachypnea or apnea and/or atypical eye movements. In general, the breathing abnormalities improve with age, truncal ataxia develops over time, and acquisition of gross motor milestones is delayed. Cognitive abilities are variable, ranging from severe intellectual disability to normal. Additional findings can include retinal dystrophy, renal disease, ocular colobomas, occipital encephalocele, hepatic fibrosis, polydactyly, oral hamartomas, and endocrine abnormalities. Both intra- and interfamilial variation are seen.

Classic Joubert syndrome (JS) is characterized by three primary findings: A distinctive cerebellar and brain stem malformation called the molar tooth sign (MTS). Hypotonia. Developmental delays. Often these findings are accompanied by episodic tachypnea or apnea and/or atypical eye movements. In general, the breathing abnormalities improve with age, truncal ataxia develops over time, and acquisition of gross motor milestones is delayed. Cognitive abilities are variable, ranging from severe intellectual disability to normal. Additional findings can include retinal dystrophy, renal disease, ocular colobomas, occipital encephalocele, hepatic fibrosis, polydactyly, oral hamartomas, and endocrine abnormalities. Both intra- and interfamilial variation are seen.

Meckel syndrome is an autosomal recessive pre- or perinatal lethal malformation syndrome characterized by renal cystic dysplasia and variably associated features including developmental anomalies of the central nervous system (typically occipital encephalocele), hepatic ductal dysplasia and cysts, and postaxial polydactyly (summary by Smith et al., 2006). For a more complete phenotypic description and information on genetic heterogeneity of Meckel syndrome, see MKS1 (249000).

Lathosterolosis (LATHOS) is an autosomal recessive disorder characterized by a recognizable pattern of multiple congenital anomalies involving axial and appendicular skeleton, liver, central nervous and urogenital systems, and lysosomal storage. It is caused by a defect of cholesterol biosynthesis due to sterol C5-desaturase deficiency (summary by Rossi et al., 2007).

Holoprosencephaly-postaxial polydactyly syndrome associates, in chromosomally normal neonates, holoprosencephaly, severe facial dysmorphism, postaxial polydactyly and other congenital abnormalities, suggestive of trisomy 13. Incidence is unknown. Dysmorphic features include hypotelorism, severe eye anomalies such as microphthalmia or anophthalmia, premaxillary region aplasia and cleft lip and palate. Congenital cardiac anomalies are common. The condition seems to be inherited as an autosomal recessive trait. Prognosis is poor.

Classic Joubert syndrome (JS) is characterized by three primary findings: A distinctive cerebellar and brain stem malformation called the molar tooth sign (MTS). Hypotonia. Developmental delays. Often these findings are accompanied by episodic tachypnea or apnea and/or atypical eye movements. In general, the breathing abnormalities improve with age, truncal ataxia develops over time, and acquisition of gross motor milestones is delayed. Cognitive abilities are variable, ranging from severe intellectual disability to normal. Additional findings can include retinal dystrophy, renal disease, ocular colobomas, occipital encephalocele, hepatic fibrosis, polydactyly, oral hamartomas, and endocrine abnormalities. Both intra- and interfamilial variation are seen.

BBS9 is an autosomal recessive disorder characterized by obesity, polydactyly, renal anomalies, retinopathy, and mental retardation (Abu-Safieh et al., 2012). For a general phenotypic description and a discussion of genetic heterogeneity of Bardet-Biedl syndrome, see BBS1 (209900).

BBS12 is a clinically pleiotropic autosomal recessive ciliopathy. The patients with BBS12 studied by Stoetzel et al. (2007) and Harville et al. (2010) met the diagnostic criteria of Beales et al. (1999), which required the presence of either 4 primary features, including rod-cone dystrophy, polydactyly, obesity, learning disabilities, hypogonadism (in males), and/or renal anomalies; or 3 primary plus 2 secondary features (e.g., developmental delay, ataxia, cataracts). For a general phenotypic description and a discussion of genetic heterogeneity of Bardet-Biedl syndrome, see BBS1 (209900).

A rare genetic congenital limb malformation disorder with characteristics of isolated, postaxial oligodactyly in all four extremities. Patients present a consistent pattern of malformation ranging from complete absence of the 5th metacarpals, metatarsals and phalanges to complete absence of the 5th metacarpals and metatarsals, with some residual distal 5th phalanges. There have been no further descriptions in the literature since 1993.

Preaxial polydactyly II (PPD2) is a limb malformation in which duplication, full or partial, of the first digital ray of hands or feet results in extra digits. Triphalangeal thumb is characterized by the presence of 3 phalanges within the thumb. The extra middle phalanx may be fully formed, trapezoidal, or a small triangular 'delta' phalanx; the thumb may be opposable or nonopposable. Preaxial polydactyly and triphalangeal thumb may cosegregate, or each occur in isolation, within families with mutation in the zone of polarizing activity (ZPA) regulatory sequence (ZRS), a regulatory element for the SHH gene (600725) that is contained within intron 5 of the LMBR1 gene (Heutink et al., 1994; Furniss et al., 2008; VanderMeer et al., 2014).

Orofaciodigital syndrome V (OFD5) is an autosomal recessive disorder characterized by cleft palate/uvula, lobulated tongue, frontal bossing, hypertelorism, postaxial polydactyly, and impaired intellectual development (summary by Faily et al., 2017).

Meckel syndrome is a disorder with severe signs and symptoms that affect many parts of the body. The most common features are enlarged kidneys with numerous fluid-filled cysts; an occipital encephalocele, which is a sac-like protrusion of the brain through an opening at the back of the skull; and the presence of extra fingers and toes (polydactyly). Most affected individuals also have a buildup of scar tissue (fibrosis) in the liver.\n\nOther signs and symptoms of Meckel syndrome vary widely among affected individuals. Numerous abnormalities of the brain and spinal cord (central nervous system) have been reported in people with Meckel syndrome, including a group of birth defects known as neural tube defects. These defects occur when a structure called the neural tube, a layer of cells that ultimately develops into the brain and spinal cord, fails to close completely during the first few weeks of embryonic development. Meckel syndrome can also cause problems with development of the eyes and other facial features, heart, bones, urinary system, and genitalia.\n\nBecause of their serious health problems, most individuals with Meckel syndrome die before or shortly after birth. Most often, affected infants die of respiratory problems or kidney failure.

This autosomal recessive disorder is designated Meckel syndrome type 7 (MKS7) based on the classic phenotypic triad of (1) cystic renal disease; (2) a central nervous system abnormality, and (3) hepatic abnormalities, as defined by Meckel (1822), Salonen (1984), and Logan et al. (2011). According to these criteria, polydactyly is a variable feature. Herriot et al. (1991) and Al-Gazali et al. (1996) concluded that Dandy-Walker malformation can be the phenotypic manifestation of a central nervous system malformation in MKS. For a general phenotypic description and a discussion of genetic heterogeneity of Meckel syndrome, see MKS1 (249000).

Because of their serious health problems, most individuals with Meckel syndrome die before or shortly after birth. Most often, affected infants die of respiratory problems or kidney failure.\n\nOther signs and symptoms of Meckel syndrome vary widely among affected individuals. Numerous abnormalities of the brain and spinal cord (central nervous system) have been reported in people with Meckel syndrome, including a group of birth defects known as neural tube defects. These defects occur when a structure called the neural tube, a layer of cells that ultimately develops into the brain and spinal cord, fails to close completely during the first few weeks of embryonic development. Meckel syndrome can also cause problems with development of the eyes and other facial features, heart, bones, urinary system, and genitalia.\n\nMeckel syndrome is a disorder with severe signs and symptoms that affect many parts of the body. The most common features are enlarged kidneys with numerous fluid-filled cysts; an occipital encephalocele, which is a sac-like protrusion of the brain through an opening at the back of the skull; and the presence of extra fingers and toes (polydactyly). Most affected individuals also have a buildup of scar tissue (fibrosis) in the liver.

Classic Joubert syndrome (JS) is characterized by three primary findings: A distinctive cerebellar and brain stem malformation called the molar tooth sign (MTS). Hypotonia. Developmental delays. Often these findings are accompanied by episodic tachypnea or apnea and/or atypical eye movements. In general, the breathing abnormalities improve with age, truncal ataxia develops over time, and acquisition of gross motor milestones is delayed. Cognitive abilities are variable, ranging from severe intellectual disability to normal. Additional findings can include retinal dystrophy, renal disease, ocular colobomas, occipital encephalocele, hepatic fibrosis, polydactyly, oral hamartomas, and endocrine abnormalities. Both intra- and interfamilial variation are seen.

Greenberg dysplasia (GRBGD), also known as hydrops-ectopic calcification-moth-eaten (HEM) skeletal dysplasia, is a rare autosomal recessive osteochondrodysplasia characterized by gross fetal hydrops, severe shortening of all long bones with a moth-eaten radiographic appearance, platyspondyly, disorganization of chondroosseous calcification, and ectopic ossification centers. It is lethal in utero. Patient fibroblasts show increased levels of cholesta-8,14-dien-3-beta-ol, suggesting a defect of sterol metabolism (summary by Konstantinidou et al., 2008). Herman (2003) reviewed the cholesterol biosynthetic pathway and 6 disorders involving enzyme defects in postsqualene cholesterol biosynthesis: Smith-Lemli-Opitz syndrome (SLOS; 270400), desmosterolosis (602398), X-linked dominant chondrodysplasia punctata (CDPX2; 302960), CHILD syndrome (308050), lathosterolosis (607330), and HEM skeletal dysplasia.

Bardet-Biedl syndrome is an autosomal recessive and genetically heterogeneous ciliopathy characterized by retinitis pigmentosa, obesity, kidney dysfunction, polydactyly, behavioral dysfunction, and hypogonadism (summary by Beales et al., 1999). Eight proteins implicated in the disorder assemble to form the BBSome, a stable complex involved in signaling receptor trafficking to and from cilia (summary by Scheidecker et al., 2014). Genetic Heterogeneity of Bardet-Biedl Syndrome BBS2 (615981) is caused by mutation in a gene on 16q13 (606151); BBS3 (600151), by mutation in the ARL6 gene on 3q11 (608845); BBS4 (615982), by mutation in a gene on 15q22 (600374); BBS5 (615983), by mutation in a gene on 2q31 (603650); BBS6 (605231), by mutation in the MKKS gene on 20p12 (604896); BBS7 (615984), by mutation in a gene on 4q27 (607590); BBS8 (615985), by mutation in the TTC8 gene on 14q32 (608132); BBS9 (615986), by mutation in a gene on 7p14 (607968); BBS10 (615987), by mutation in a gene on 12q21 (610148); BBS11 (615988), by mutation in the TRIM32 gene on 9q33 (602290); BBS12 (615989), by mutation in a gene on 4q27 (610683); BBS13 (615990), by mutation in the MKS1 gene (609883) on 17q23; BBS14 (615991), by mutation in the CEP290 gene (610142) on 12q21, BBS15 (615992), by mutation in the WDPCP gene (613580) on 2p15; BBS16 (615993), by mutation in the SDCCAG8 gene (613524) on 1q43; BBS17 (615994), by mutation in the LZTFL1 gene (606568) on 3p21; BBS18 (615995), by mutation in the BBIP1 gene (613605) on 10q25; BBS19 (615996), by mutation in the IFT27 gene (615870) on 22q12; BBS20 (619471), by mutation in the IFT172 gene (607386) on 9p21; BBS21 (617406), by mutation in the CFAP418 gene (614477) on 8q22; and BBS22 (617119), by mutation in the IFT74 gene (608040) on 9p21. The CCDC28B gene (610162) modifies the expression of BBS phenotypes in patients who have mutations in other genes. Mutations in MKS1, MKS3 (TMEM67; 609884), and C2ORF86 also modify the expression of BBS phenotypes in patients who have mutations in other genes. Although BBS had originally been thought to be a recessive disorder, Katsanis et al. (2001) demonstrated that clinical manifestation of some forms of Bardet-Biedl syndrome requires recessive mutations in 1 of the 6 loci plus an additional mutation in a second locus. While Katsanis et al. (2001) called this 'triallelic inheritance,' Burghes et al. (2001) suggested the term 'recessive inheritance with a modifier of penetrance.' Mykytyn et al. (2002) found no evidence of involvement of the common BBS1 mutation in triallelic inheritance. However, Fan et al. (2004) found heterozygosity in a mutation of the BBS3 gene (608845.0002) as an apparent modifier of the expression of homozygosity of the met390-to-arg mutation in the BBS1 gene (209901.0001). Allelic disorders include nonsyndromic forms of retinitis pigmentosa: RP51 (613464), caused by TTC8 mutation, and RP55 (613575), caused by ARL6 mutation.

BBS2 is an autosomal recessive ciliopathy characterized by retinal degeneration, polydactyly, renal disease, hypogonadism, obesity, dysmorphic features, and variable degrees of cognitive impairment (Innes et al., 2010). Mutation in the BBS2 gene is the third most frequent cause of BBS, accounting for approximately 8% of cases (Zaghloul and Katsanis, 2009). For a general phenotypic description and a discussion of genetic heterogeneity of Bardet-Biedl syndrome, see BBS1 (209900).

Hydrolethalus syndrome is an autosomal recessive embryonic lethal disorder characterized by hydrocephaly or anencephaly, postaxial polydactyly of the upper limbs, and pre- or postaxial polydactyly of the lower limbs. Duplication of the hallux is a common finding. HLS2 is considered a ciliopathy (summary by Putoux et al., 2011). Acrocallosal syndrome (ACLS; 200990) is an allelic disorder with a less severe phenotype. For a discussion of genetic heterogeneity of hydrolethalus syndrome, see 236680.

Meckel syndrome is a disorder with severe signs and symptoms that affect many parts of the body. The most common features are enlarged kidneys with numerous fluid-filled cysts; an occipital encephalocele, which is a sac-like protrusion of the brain through an opening at the back of the skull; and the presence of extra fingers and toes (polydactyly). Most affected individuals also have a buildup of scar tissue (fibrosis) in the liver.\n\nOther signs and symptoms of Meckel syndrome vary widely among affected individuals. Numerous abnormalities of the brain and spinal cord (central nervous system) have been reported in people with Meckel syndrome, including a group of birth defects known as neural tube defects. These defects occur when a structure called the neural tube, a layer of cells that ultimately develops into the brain and spinal cord, fails to close completely during the first few weeks of embryonic development. Meckel syndrome can also cause problems with development of the eyes and other facial features, heart, bones, urinary system, and genitalia.\n\nBecause of their serious health problems, most individuals with Meckel syndrome die before or shortly after birth. Most often, affected infants die of respiratory problems or kidney failure.

Meckel syndrome, also known as Meckel-Gruber syndrome, is a severe pleiotropic autosomal recessive developmental disorder caused by dysfunction of primary cilia during early embryogenesis. There is extensive clinical variability and controversy as to the minimum diagnostic criteria. Early reports, including that of Opitz and Howe (1969) and Wright et al. (1994), stated that the classic triad of Meckel syndrome comprises (1) cystic renal disease; (2) a central nervous system malformation, most commonly occipital encephalocele; and (3) polydactyly, most often postaxial. However, based on a study of 67 patients, Salonen (1984) concluded that the minimum diagnostic criteria are (1) cystic renal disease; (2) CNS malformation, and (3) hepatic abnormalities, including portal fibrosis or ductal proliferation. In a review of Meckel syndrome, Logan et al. (2011) stated that the classic triad first described by Meckel (1822) included occipital encephalocele, cystic kidneys, and fibrotic changes to the liver. Genetic Heterogeneity of Meckel Syndrome See also MKS2 (603194), caused by mutation in the TMEM216 gene (613277) on chromosome 11q12; MKS3 (607361), caused by mutation in the TMEM67 gene (609884) on chromosome 8q; MKS4 (611134), caused by mutation in the CEP290 gene (610142) on chromosome 12q; MKS5 (611561), caused by mutation in the RPGRIP1L gene (610937) on chromosome 16q12; MKS6 (612284), caused by mutation in the CC2D2A gene (612013) on chromosome 4p15; MKS7 (267010), caused by mutation in the NPHP3 (608002) gene on chromosome 3q22; MKS8 (613885), caused by mutation in the TCTN2 gene (613846) on chromosome 12q24; MKS9 (614209), caused by mutation in the B9D1 gene (614144) on chromosome 17p11; MKS10 (614175), caused by mutation in the B9D2 gene (611951) on chromosome 19q13; MKS11 (615397), caused by mutation in the TMEM231 gene (614949) on chromosome 16q23; MKS12 (616258), caused by mutation in the KIF14 gene (611279) on chromosome 1q32; MKS13 (617562), caused by mutation in the TMEM107 gene (616183) on chromosome 17p13; and MKS14 (619879), caused by mutation in the TXNDC15 gene (617778) on chromosome 5q31.

Bardet-Biedl syndrome-17 (BBS17) is an autosomal recessive ciliopathy characterized by retinitis pigmentosa, cognitive impairment, obesity, renal dysfunction, and hypogenitalism. Polydactyly, most often postaxial, is also a primary feature of BBS; in BBS17, mesoaxial polydactyly, with fused or Y-shaped metacarpals, is a distinct manifestation (Deffert et al., 2007; Schaefer et al., 2014). For a general phenotypic description and a discussion of genetic heterogeneity of Bardet-Biedl syndrome, see BBS1 (209900).

Classic Joubert syndrome (JS) is characterized by three primary findings: A distinctive cerebellar and brain stem malformation called the molar tooth sign (MTS). Hypotonia. Developmental delays. Often these findings are accompanied by episodic tachypnea or apnea and/or atypical eye movements. In general, the breathing abnormalities improve with age, truncal ataxia develops over time, and acquisition of gross motor milestones is delayed. Cognitive abilities are variable, ranging from severe intellectual disability to normal. Additional findings can include retinal dystrophy, renal disease, ocular colobomas, occipital encephalocele, hepatic fibrosis, polydactyly, oral hamartomas, and endocrine abnormalities. Both intra- and interfamilial variation are seen.

Bardet-Biedl syndrome-19 (BBS19) is an autosomal recessive ciliopathy characterized by obesity, impaired intellectual development, polydactyly, renal failure, retinitis pigmentosa, and hypogonadism (Aldahmesh et al., 2014). For a general phenotypic description and a discussion of genetic heterogeneity of Bardet-Biedl syndrome, see BBS1 (209900).

Classic Joubert syndrome (JS) is characterized by three primary findings: A distinctive cerebellar and brain stem malformation called the molar tooth sign (MTS). Hypotonia. Developmental delays. Often these findings are accompanied by episodic tachypnea or apnea and/or atypical eye movements. In general, the breathing abnormalities improve with age, truncal ataxia develops over time, and acquisition of gross motor milestones is delayed. Cognitive abilities are variable, ranging from severe intellectual disability to normal. Additional findings can include retinal dystrophy, renal disease, ocular colobomas, occipital encephalocele, hepatic fibrosis, polydactyly, oral hamartomas, and endocrine abnormalities. Both intra- and interfamilial variation are seen.

Short-rib thoracic dysplasia (SRTD) with or without polydactyly refers to a group of autosomal recessive skeletal ciliopathies that are characterized by a constricted thoracic cage, short ribs, shortened tubular bones, and a 'trident' appearance of the acetabular roof. SRTD encompasses Ellis-van Creveld syndrome (EVC) and the disorders previously designated as Jeune syndrome or asphyxiating thoracic dystrophy (ATD), short rib-polydactyly syndrome (SRPS), and Mainzer-Saldino syndrome (MZSDS). Polydactyly is variably present, and there is phenotypic overlap in the various forms of SRTDs, which differ by visceral malformation and metaphyseal appearance. Nonskeletal involvement can include cleft lip/palate as well as anomalies of major organs such as the brain, eye, heart, kidneys, liver, pancreas, intestines, and genitalia. Some forms of SRTD are lethal in the neonatal period due to respiratory insufficiency secondary to a severely restricted thoracic cage, whereas others are compatible with life (summary by Huber and Cormier-Daire, 2012 and Schmidts et al., 2013). There is phenotypic overlap with the cranioectodermal dysplasias (Sensenbrenner syndrome; see CED1, 218330). For a discussion of genetic heterogeneity of short-rib thoracic dysplasia with or without polydactyly, see SRTD1 (208500).

Postaxial polydactyly type A7 (PAPA7) is an autosomal recessive disorder characterized by postaxial polydactyly and brachydactyly of the hands and/or feet. Features reported in some patients include syndactyly of the second and third digits of the feet, learning disabilities, and increased body weight (Umair et al., 2017; Estrada-Cuzcano et al., 2020). For a discussion of genetic heterogeneity of postaxial polydactyly, see 174200.

Joubert syndrome-32 (JBTS32) is an autosomal recessive developmental disorder characterized by delayed psychomotor development, intellectual disability, dysmorphic facial features, and postaxial polydactyly. Brain imaging shows cerebellar abnormalities consistent with the molar tooth sign (MTS) (summary by De Mori et al., 2017). For discussion of genetic heterogeneity of Joubert syndrome, see JBTS1 (213300).

Classic Joubert syndrome (JS) is characterized by three primary findings: A distinctive cerebellar and brain stem malformation called the molar tooth sign (MTS). Hypotonia. Developmental delays. Often these findings are accompanied by episodic tachypnea or apnea and/or atypical eye movements. In general, the breathing abnormalities improve with age, truncal ataxia develops over time, and acquisition of gross motor milestones is delayed. Cognitive abilities are variable, ranging from severe intellectual disability to normal. Additional findings can include retinal dystrophy, renal disease, ocular colobomas, occipital encephalocele, hepatic fibrosis, polydactyly, oral hamartomas, and endocrine abnormalities. Both intra- and interfamilial variation are seen.

Postaxial polydactyly type A9 is characterized by one or more posterior or postaxial digits. There is intrafamilial and intraindividual variability (Schrauwen et al., 2018).

Postaxial polydactyly type A10 (PAPA10) is characterized by one or more postaxial digits of the hands and/or feet. A rudimentary digit (PAP type B) may also be present. Intrafamilial variability has been observed (Ullah et al., 2019). For a discussion of genetic heterogeneity of postaxial polydactyly, see 174200.

Cardioacrofacial dysplasia-2 (CAFD2) is characterized by congenital cardiac defects, primarily common atrium or atrioventricular septal defect; limb anomalies, including short limbs, brachydactyly, and postaxial polydactyly; and dysmorphic facial features. Developmental delay of variable severity has also been observed (Palencia-Campos et al., 2020). For a discussion of genetic heterogeneity of CAFD, see CAFD1 (619142).

X-linked multiple congenital anomalies-neurodevelopmental syndrome (MCAND) is an X-linked recessive congenital multisystemic disorder characterized by poor growth, global developmental delay with impaired intellectual development, and variable abnormalities of the cardiac, skeletal, and genitourinary systems. Most affected individuals also have hypotonia and dysmorphic craniofacial features. Brain imaging typically shows enlarged ventricles and thin corpus callosum; some have microcephaly, whereas others have hydrocephalus. The severity of the disorder is highly variable, ranging from death in early infancy to survival into the second or third decade. Pathogenetically, the disorder results from disrupted gene expression and signaling during embryogenesis, thus affecting multiple systems (summary by Tripolszki et al., 2021 and Beck et al., 2021). Beck et al. (2021) referred to the disorder as LINKED syndrome (LINKage-specific deubiquitylation deficiency-induced Embryonic Defects).

Bardet-Biedl syndrome-22 (BBS22) is an autosomal recessive ciliopathy described in a single patient and characterized by retinitis pigmentosa, obesity, polydactyly, hypogonadism, and intellectual disability (Lindstrand et al., 2016). For a general phenotypic description and a discussion of genetic heterogeneity of Bardet-Biedl syndrome, see BBS1 (209900).

Synpolydactyly (SPD), or syndactyly type II, is defined as a connection between the middle and ring fingers and fourth and fifth toes, variably associated with postaxial polydactyly in the same digits. Minor local anomalies and various metacarpal or metatarsal abnormalities may be present (summary by Merlob and Grunebaum, 1986). In some families with SPD, the foot anomalies are characterized by preaxial as well as postaxial polydactyly, and appear to be fully penetrant. The more severe features of classic SPD, involving 3/4 synpolydactyly in the hands and 4/5 synpolydactyly in the feet, also occur, but at reduced penetrance. This foot phenotype is not seen in patients with classic SPD due to HOXD13 polyalanine tract expansions (Goodman et al., 1998). Malik (2012) reviewed the syndactylies, noting that the extreme phenotypic heterogeneity observed in SPD families consists of approximately 18 clinical variants that can be 'lumped' into 3 categories: typical SPD features, minor variants, and unusual phenotypes. Genetic Heterogeneity of Synpolydactyly See also SPD2 (608180), caused by mutation in the fibulin-1 gene (FBLN1; 135820) on chromosome 22q13, and SPD3 (610234), which has been mapped to chromosome 14q11.2-q12.

Meckel syndrome-14 (MKS14) is a lethal disorder characterized by occipital encephalocele, postaxial polydactyly of the hands and feet, and polycystic kidneys. Stillbirth has been reported, as well as death within hours in a live-born affected individual (Shaheen et al., 2016; Ridnoi et al., 2019). For a general phenotypic description and discussion of genetic heterogeneity of Meckel syndrome, see MKS1 (249000).

Orofaciodigital syndrome XIX (OFD19) is an autosomal recessive ciliopathy characterized by tongue nodules; dental anomalies including congenital absence or abnormal shape of incisors; narrow, high-arched or cleft palate; retrognathia; and digital anomalies. Some patients have notching of the upper or lower lip (Iturrate et al., 2022).