Defects in the assembly of dolichol-linked oligosaccharide or its transfer to proteins result in severe, multi-system human diseases called Type I congenital disorders of glycosylation. We have identified a novel CDG type, CDG-Ij, resulting from deficiency in UDP-GlcNAc: dolichol phosphate N-acetyl-glucosamine-1 phosphate transferase (GPT) activity encoded by DPAGT1. The patient presents with severe hypotonia, medically intractable seizures, mental retardation, microcephaly, and exotropia. Metabolic labeling of cultured dermal fibroblasts from the patient with [2-(3)H]-mannose revealed lowered incorporation of radiolabel into full-length dolichol-linked oligosaccharides and glycoproteins. In vitro enzymatic analysis of microsomal fractions from the cultured cells indicated that oligosaccharyltransferase activity is normal, but the GPT activity is reduced to approximately 10% of normal levels while parents have heterozygous levels. The patient's paternal DPAGT1 allele contains a point mutation (660A>G) that replaces a highly conserved tyrosine with a cysteine (Y170C). The paternal allele cDNA produces a full-length protein with almost no activity when over-expressed in CHO cells. The maternal allele makes only about 12% normal mature mRNA, while the remainder shows a complex exon skipping pattern that shifts the reading frame encoding a truncated non-functional GPT protein. Thus, we conclude that the DPAGT1 gene defects are responsible for the CDG symptoms in this patient. Hum Mutat 22:144-150, 2003.