Dolichyl pyrophosphate mannosyltransferases add mannosyl residues to the 'chitobiose core' in the endoplasmic reticulum membrane during the process of protein N-glycosylation. The chitobiose core is composed of two N-acetylglucosamine (GlcNAc) residues attached to the dolichol pyrophosphate (PP-Dol) lipid carrier ((GlcNAc[[2]]-PP-Dol)). Alg1, Alg2 and Alg11 enzymes act first to add extend this core with 5 mannose residues to form the Man[[5]]GlcNAc[[2]]-PP-Dol intermediate. The Alg3, Alg12 and Alg9 enzymes then further elongate this to form Man[[9]]GlcNAc[[2]]-PP-Dol. Following a final glucosylation step, the finished precursor oligosaccharide (Glc[[3]]Man[[9]]GlcNAc[[2]]-PP-Dol) is transferred to nascent peptides, where it will be further trimmed down and modified to produce the final glycoprotein. (Adapted from FBrf0235108.)