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-T12 (Polypeptide N-acetylgalactosaminyltransferase 12), also known as UDP-GalNAc:polypeptide N-acetylgalactosaminyltransferase 12, is a 581 amino acid protein that displays enzymatic activity towards non-glycosylated peptides such as Muc5Ac, Muc1a and EA2 with no detectable activity towards Muc2 and Muc7. The N-terminal domain is involved in substrate binding and manganese coordination, while the C-terminal domain is involved in UDP-Gal binding and catalytic reaction. Since GalNAc-T12 is highly expressed in stomach, pancreas, small intestine and colon, it may play a significant role in the initial step of mucin-type oligosaccharide biosynthesis in digestive organs.