The UDP-N-acetyl-alpha-D-galactosamine:polypeptide N-acetylgalactosaminyltransferase (GalNAc-T) family of enzymes are substrate-specific proteins that catalyze the transfer of GalNAc (N-acetylgalactosamine) to serine and threonine residues onto various proteins, thereby initiating mucin-type O-linked glycosylation in the Golgi apparatus. GalNAc-T1, also known as GALNT1 (Polypeptide N-acetylgalactosaminyltransferase 1), is a ubiquitously expressed 559 amino acid single-pass type II membrane protein that localizes to the Golgi apparatus and, like other GalNAc-Ts, contains a stem region and a C-terminal ricin lectin-like domain. GalNAc-T1 catalyzes the first reaction in O-linked oligosaccharide biosynthesis, namely the transfer of an N-acetyl-D-galactosamine residue to a protein acceptor. GalNAc-T1 uses calcium and manganese as cofactors. Due to alternative splicing events, two GalNAc-T1 isoforms are expressed.