GalNAc-T3 deficiency is a rare congenital condition resulting in the disease familial tumoral calcinosis characterized by hyperphosphatemia and ectopic ossifications and caused by a lack of GalNAc-T3-mediated site-specific O-glycosylation of FGF23 that protects it from inactivating proprotein convertase processing (12, 13). The gene discussed is FGF23; the disease is hyperphosphatemia.