(A) The enzyme MGAT2 catalyzes the attachment of a GlcNAc donor monosaccharide to a mannose acceptor monosaccharide. Black arrows show monosaccharide addition reactions. The enzyme makes specific contacts with multiple moieties of the acceptor, which we term its ‘recognition motif’ (dotted red oval/box). The protein structure is based on PDB id: 5VCS; the position of the donor monosaccharide is modeled. (B) In eukaryotes, glycan synthesis by glycosyltransferase enzymes takes place within membrane-bounded reaction compartments (black boxes) such as the lumen of the ER or the cisternae of the Golgi apparatus. Growing oligosaccharides can exit the reaction compartment (red arrows). We highlight the three enzymatic causes of glycan microheterogeneity: truncated, runaway and divergent reactions.