an enzyme that 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. A single-pass type II membrane protein that localizes to the Golgi apparatus membrane. Has activity toward HIV envelope glycoprotein gp120, EA2, Muc2 and Muc5. Probably glycosylates fibronectin in vivo. Plays a central role in phosphate homeostasis. Mutations cause cause familial tumoral calcinosis, a severe autosomal recessive metabolic disorder that manifests with hyperphosphatemia and massive calcium deposits in the skin and subcutaneous tissues. Overexpressed in many differentiated carcinomas, suggesting that it may serve as a marker of tumor differentiation. Two alternatively spliced human isoforms have been described. Note: This description may include information from UniProtKB.