Synthesis of the dolichol-linked oligosaccharide (LLO) precursor is the first step in protein N-glycosylation. LLO is composed of a lipid carrier, dolichyl pyrophosphate, to which is attached a glycan containing 2 N-acetylglucosamine (GlcNAc), 9 mannose (Man) and 3 glucose (Glc) residues. LLO synthesis begins on the cytosolic side of the endoplasmic reticulum (ER) membrane and terminates in the ER lumen where the oligosaccharyl transferase (OST) complexes transfer the oligosaccharide to asparagine residues within a N-X-S/T sequon of a nascent protein co- and post-translationally. (Adapted from FBrf0235108.)