Catalyzes the initial reaction in O-linked oligosaccharide biosynthesis, the transfer of an N-acetyl-D-galactosamine residue to a serine or threonine residue on the protein receptor. Has activity toward non-glycosylated peptides such as Muc5AC, Muc1a and EA2, and no detectable activity with Muc2 and Muc7. Displays enzymatic activity toward the Gal-NAc-Muc5AC glycopeptide, but no detectable activity to mono-GalNAc-glycosylated Muc1a, Muc2, Muc7 and EA2. May play an important role in the initial step of mucin-type oligosaccharide biosynthesis in digestive organs. L-seryl-[protein] + UDP-N-acetyl-alpha-D-galactosamine = 3-O-[N-acetyl-alpha-D-galactosaminyl]-L-seryl-[protein] + H(+) + UDP L-threonyl-[protein] + UDP-N-acetyl-alpha-D-galactosamine = 3-O-[N-acetyl-alpha-D-galactosaminyl]-L-threonyl-[protein] + H(+) + UDP Protein modification; protein glycosylation. Widely expressed at different levels of expression. Highly expressed in digestive organs such as small intestine, stomach, pancreas and colon. Expressed at intermediate level in testis, thyroid gland and spleen. Weakly expressed in whole brain, cerebral cortex, cerebellum, fetal brain, bone marrow, thymus, leukocytes, heart, skeletal muscle, liver, lung, esophagus, kidney, adrenal gland, mammary gland, uterus, placenta, ovary and prostate. There are two conserved domains in the glycosyltransferase region: the N-terminal domain (domain A, also called GT1 motif), which is probably involved in manganese coordination and substrate binding and the C-terminal domain (domain B, also called Gal/GalNAc-T motif), which is probably involved in catalytic reaction and UDP-Gal binding. The ricin B-type lectin domain binds to GalNAc and contributes to the glycopeptide specificity. Disease susceptibility is associated with variants affecting the gene represented in this entry. The role of GALNT12 in colon cancer susceptibility is however subject to discussion: studies on 103 probants with colorectal cancer 1 (CRCS1) suggest that it does not act as a major contributor of CRCS1 (PubMed:24115450). Belongs to the glycosyltransferase 2 family. GalNAc-T subfamily. Polypeptide N-acetylgalactosaminyltransferase 12