FGF23 is a small circulating hormone, primarily secreted by osteocytes, that balances mineral ion homeostasis by acting upon regulatory mechanisms in the kidney that control renal phosphate excretion.30 FGF23 also controls OPN levels.31 It is implicated in the pathogenesis of various renal phosphate-wasting diseases, including X-linked hypophosphatemia (XLH), autosomal dominant hypophosphatemic rickets (ADHR), oncogenic osteomalacia (OOM), chronic kidney disease (CKD), familial tumoral calcinosis (FTC), McCune–Albright syndrome, and fibrous dysplasia of bone. The gene discussed is FGF23; the disease is X-linked hypophosphatemia.