A new type of congenital disorders of glycosylation, designated CDG-Ip, is caused by the deficiency of GDP-Man:Man3GlcNAc2-PP-dolichol-alpha1,2-mannosyltransferase, encoded by the human ortholog of ALG11 from yeast. The patient presented with a multisystemic disorder characterized by muscular hypotonia, seizures, developmental retardation and death at the age of 2 years. The isoelectric focusing pattern of the patient's serum transferrin showed the partial loss of complete N-glycan side chains, which is a characteristic sign for CDG-I. Analysis of dolichol-linked oligosaccharides in patient-derived fibroblasts revealed an accumulation of Man3GlcNAc2-PP-dolichol and Man4GlcNAc2-PP-dolichol. Determination of mannosyltransferase activities of early steps of lipid-linked oligosaccharide biosynthesis in fibroblasts indicated that the patient was deficient in elongating Man3GlcNAc2-PP-dolichol. These findings gave rise to genetic analysis of the hALG11 cDNA, in which homozygosity for mutation c.T257C (p.L86S) was identified. Verification of the mutation as a primary cause for the genetic defect was proved by retroviral expression of human wild-type and mutated ALG11 cDNA in patient-derived fibroblasts as well as using a yeast alg11 deletion strain as a heterologous expression system for hALG11 variants. Immunofluorescence examinations combined with western blotting showed no differences of intracellular localization or expression of ALG11 between control and patient fibroblasts, respectively, indicating no mislocalization or degradation of the mutated transferase.