Polypeptide N-acetylgalactosaminyltransferase 3 (GalNAc-T3) catalyzes the transfer of N-acetylgalactosamine (GalNAc) from UDP-GalNAc to the hydroxyl group of serine and threonine residues. Twenty GalNAc-T genes have been identified in humans and most have been shown to represent active GalNAc-Ts. All isoforms are type II transmembrane proteins, with different but partly overlapping substrate preferences. The GalNAc-Ts control the initiation of mucin-type O-linked glycosylation and determine the location and density of O-glycans in a protein. Addition of GalNAc to an unglycosylated Ser/Thr residue creates the Tn antigen GalNAcα1-S/T, and subsequent addition of sialic acid by ST6GalNAc-I forms the cancer associated STn antigen. GalNAc-T