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Congenital disorders of glycosylation (CDGs) are metabolic deficiencies in glycoprotein biosynthesis that usually cause severe mental and psychomotor retardation. Different forms of CDGs can be recognized by altered isoelectric focusing (IEF) patterns of serum transferrin (Tf). Two patients with these symptoms and similar abnormal TfIEF patterns were analyzed by metabolic labeling of fibroblasts with [2-3H]mannose. The patients produced a truncated dolichol-linked precursor oligosaccharide with 5 mannose residues, instead of the normal precursor with 9 mannose residues. Addition of 250 μM mannose to the culture medium corrected the size of the truncated oligosaccharide. Microsomes from fibroblasts of these patients were approximately 95% deficient in dolichol-phosphate-mannose (Dol-P-Man) synthase activity, with an apparent Km for GDP-Man ∼6-fold higher than normal. DPM1, the gene coding for the catalytic subunit of Dol-P-Man synthase, was altered in both patients. One patient had a point mutation, C274G, causing an R92G change in the coding sequence. The other patient also had the C274G mutation and a 13-bp deletion that presumably resulted in an unstable transcript. Defects in DPM1 define a new glycosylation disorder, CDG-Ie.