Hyp mice with inactivating Phex mutations have very high production of FGF23 in osteocytes and markedly elevated circulating FGF23 levels leading to hypophosphatemia and abnormalities in vitamin D metabolism, as well as impaired mineralization of cartilage and bone leading to rickets and osteomalacia [31], [39], [47]. Here, FGF23 is linked to osteomalacia.