ENPP1 and osteomalacia: Hypophosphatemic rickets/osteomalacia related to an excessive action of FGF23 is also caused by inactivating mutations in the phosphate-regulating gene with homologies to endopeptidases, on the X chromosome (PHEX), dentin matrix protein 1 (DMP1), ectonucleotide pyrophosphatase/phosphodiesterase 1 (ENPP1), and family with sequence similarity 20 C (FAM20C) genes (48, 50–55).