The function of GALNT9 (polypeptide N-acetylgalactosaminyltransferase 9, ENSG00000182870) 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. Does not glycosylate apomucin or SDC3. {ECO:0000269|PubMed:10978536, ECO:0000269|PubMed:12407114}.