Gene dosage effects for soluble isocitrate dehydrogenase (IDH1) were investigated in four unrelated cases with abnormalities involving the long arm of chromosome 2. Case 1 was trisomic for 2q33.3----qter, Case 2 monosomic for 2q33.3----q35, Case 3 trisomic for 2q11.2----q24.2, and Case 4 monosomic for 2q23----q24.2. These abnormalities were de novo except in Case 1, where trisomy 2q resulted from a maternal translocation. The red cell IDH1 levels were significantly reduced in Cases 1 (41.4% of normal value) and 2 (51.9%), while they were normal in Cases 3 and 4. The low IDH1 level also in the father of Case 1 (43.6%), together with the common electrophoretic phenotype of IDH1 in red cells as well as leukocytes, led us to suppose that Case 1 was really heterozygous for common and probable null alleles, and that the IDH1 gene locus could be excluded from 2q33.3----qter. On the other hand, normal IDH1 values in the parents of Case 2 were consistent with the hemizygosity for this locus in Case 2. The results suggested that the IDH1 locus could be assigned to the 2q33.3 band, especially the proximal portion of it.