The protein encoded by this gene is a membrane-bound polypeptide N-acetylgalactosaminyltransferase that is found in the Golgi. The encoded protein catalyzes the first step in the mucin-type O-glycosylation of Golgi proteins, transfering an N-acetyl-D-galactosamine residue to a serine or threonine residue on the protein receptor. [provided by RefSeq, Aug 2016].