UDP-GlcNAc:dolichol-P GlcNAc-1-P transferase (GPT) catalyzes the transfer of GlcNAc-1-P from UDP-GlcNAc to dolichol-P to form GlcNAc-P-P-dolichol. The reaction is the first step in the assembly of dolichol-linked oligosaccharide intermediates and is essential for eukaryotic N-glycosylation. GPT activity has been identified in all eukaryotic cells examined to date. A series of six conserved motifs designated A through F, ranging in length from 5 to 13 amino acid residues, has been identified in this family. They have been determined to be important for stable expression, substrate binding, or catalytic activities.