Deficiency of the endoplasmic reticulum enzyme dolichyl-phosphate mannose (Dol-P-Man):Man(7)GlcNAc(2)-PP-dolichyl mannosyltransferase leads to a new type of congenital disorder of glycosylation, designated type Ig. The patient 1 presented with a multisystemic disorder with microcephaly, developmental retardation, convulsions and dysmorphic signs. The isoelectric focusing pattern of the patient's serum transferrin showed the partial loss of complete N-glycan side chains. In skin fibroblasts from the patient, the activity of Dol-P-Man:Man(7)GlcNAc(2)-PP-Dol mannosyltransferase was severely reduced leading to the accumulation of Man(7)GlcNAc(2)-PP-Dol, which was transferred to newly synthesized glycoproteins. Sequencing of the Dol-P-Man:Man(7)GlcNAc(2)-PP-Dol mannosyltransferase cDNA revealed a compound heterozygosity for two point mutations, leading to the exchange of leucine(158) for a proline residue and a premature translation stop with loss of the C-terminal 74 amino acids. The parents were heterozygous for one of the two mutations. Retroviral expression of the wild-type Dol-P-Man:Man(7)GlcNAc(2)-PP-Dol mannosyltransferase cDNA in patient's fibroblasts normalized the mannosyltransferase activity.