The function of Ensembl gene identifier ENSG00000115339 (GALNT3, polypeptide N-acetylgalactosaminyltransferase 3) is as follows. 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 (PubMed:16638743, PubMed:31932717, PubMed:8663203, PubMed:9295285). Has activity toward HIV envelope glycoprotein gp120, EA2, MUC2, MUC1A and MUC5AC (PubMed:8663203, PubMed:9295285). Probably glycosylates fibronectin in vivo (PubMed:9295285). Glycosylates FGF23 (PubMed:16638743, PubMed:31932717). {ECO:0000269|PubMed:16638743, ECO:0000269|PubMed:31932717, ECO:0000269|PubMed:8663203, ECO:0000269|PubMed:9295285}.