GALNT3 (Polypeptide N-Acetylgalactosaminyltransferase 3): GALNT3 is a gene that encodes a protein involved in O-linked glycosylation, adding N-acetylgalactosamine to proteins. This modification is important for proper protein function and cell signaling. Mutations in GALNT3 can affect calcium and phosphate balance, leading to disorders such as familial tumoral calcinosis.