GalNAc-T17 is a 601 amino acid single-pass type II membrane protein that is a member of the glycosyltransferase 2 family and GalNAc-T subfamily. Localized to the Golgi apparatus and contains a ricin B-type lectin domain, GalNAc-T17 catalyzes the initial reaction in O-linked oligosaccharide biosynthesis, the transfer of an N-acetyl-D-galactosamine residue to a serine or threonine residue on the protein receptor. GalNAc-T17 contains two conserved domains, an N-terminal domain (domain A, also called GT1 motif), which is likely involved in manganese coordination and substrate binding and a C-terminal domain (domain B, also called Gal/GalNAc-T motif), which is likely involved in catalytic reaction and UDP-Gal binding. GalNAc-T17 exists as two alternatively spliced isoforms and utilizes manganese and calcium as cofactors.