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-acetylgalactosaminyl) to serine and threonine residues onto various proteins, thereby initiating mucin-type O-linked glycosylation in the Golgi apparatus. GalNAc-TL1 (UDP-N-acetyl-alpha-D-galactosamine:polypeptide N-acetylgalactosaminyltransferase-like 1), also known as GALNT16, is a 588 amino acid single-pass type II membrane protein belonging to the glycosyltransferase 2 family, which localizes to the Golgi apparatus. GalNAc-TL1 utilizes manganese and calcium as cofactors, and catalyzes the initial reaction in O-linked oligosaccharide biosynthesis, which involves the transfer of an N-acetyl-D-galactosamine residue to a serine or threonine residue on the protein receptor. Containing one ricin B-type lectin domain, GalNAc-TL1 exists as two alternatively spliced isoforms.