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