Mannose-phosphate-dolichol utilization defect 1 (MPDU1) plays a role in the utilization of dolichol-phosphate-mannose (DPM). CDG-If is caused by a defect in the gene MPDU1, the human homolog of hamster Lec35, and is the first disorder to affect the use, rather than the biosynthesis, of donor substrates for lipid-linked oligosaccharides. This leads to the synthesis of incomplete and poorly transferred precursor oligosaccharides lacking both mannose and glucose residues. Kato III cells lacking MPDU1 have all the hallmarks of a glycosylation-deficient cell line, including altered sensitivity to lectins and the formation of truncated Lipid-linked Oligosaccharides (LLOs). Kato III cells and the MPDU1-rescued Kato IIIM cells provide a novel model to examine the consequences of defective LLO biosynthesis both in vitro and in vivo.