Carbohydrate-deficient glycoprotein syndrome (CDGS) represents a class of genetic diseases characterized by abnormal N-linked glycosylation. CDGS patients show a large number of glycoprotein abnormalities resulting in dysmorphy, encephalopathy, and other organ disorders. The majority of CDGSs described to date are related to an impaired biosynthesis of dolichyl pyrophosphate-linked Glc₃Man₉GlcNAc₂ in the endoplasmic reticulum. Recently, we identified in four related patients a novel type of CDGS characterized by an accumulation of dolichyl pyrophosphate-linked Man₉GlcNAc₂. Elaborating on the analogy of this finding with the phenotype of alg5 and alg6 Saccharomyces cerevisiae strains, we have cloned and analyzed the human orthologs to the ALG5 dolichyl phosphate glucosyltransferase and ALG6 dolichyl pyrophosphate Man₉GlcNAc₂ α1,3-glucosyltransferase in four novel CDGS patients. Although ALG5 was not altered in the patients, a C→T transition was detected in ALG6 cDNA of all four CDGS patients. The mutation cosegregated with the disease in a Mendelian recessive manner. Expression of the human ALG5 and ALG6 cDNA could partially complement the respective S. cerevisiae alg5 and alg6 deficiency. By contrast, the mutant ALG6 cDNA of CDGS patients failed to revert the hypoglycosylation observed in alg6 yeasts, thereby proving a functional relationship between the alanine to valine substitution introduced by the C→T transition and the CDGS phenotype. The mutation in the ALG6 α1,3-glucosyltransferase gene defines an additional type of CDGS, which we propose to refer to as CDGS type-Ic.