N-linked (asparagine-linked) glycosylation of proteins is mediated by a highly conserved pathway in eukaryotes, in which a lipid (dolichol phosphate)-linked oligosaccharide is assembled at the endoplasmic reticulum membrane prior to the transfer of the oligosaccharide moiety to the target asparagine residues. This oligosaccharide is composed of Glc(3)Man(9)GlcNAc(2). The addition of the three glucose residues is the final series of steps in the synthesis of the oligosaccharide precursor. Alg6 transfers the first glucose residue, and Alg8 transfers the second one. In the human alg6 gene, a C->T transition, which causes Ala333 to be replaced with Val, has been identified as the cause of a congenital disorder of glycosylation, designated as type Ic OMIM:603147.