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 HIV envelope glycoprotein gp120, EA2, Muc2 and Muc5. Probably glycosylates fibronectin in vivo. Glycosylates FGF23. Plays a central role in phosphate homeostasis. 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. Resides preferentially in the trans and medial parts of the Golgi stack. Expressed in organs that contain secretory epithelial glands. Highly expressed in pancreas, skin, kidney and testis. Weakly expressed in prostate, ovary, intestine and colon. Also expressed in placenta and lung and fetal lung and fetal kidney. 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. The disease is caused by variants affecting the gene represented in this entry. Overexpressed in many differentiated carcinomas, suggesting that it may serve as a marker of tumor differentiation. Belongs to the glycosyltransferase 2 family. GalNAc-T subfamily. Polypeptide N-acetylgalactosaminyltransferase 3