ENPP1 and hypophosphatemic rickets: Hypophosphatemic rickets and elevated serum FGF23 levels including XLH [6, 8], autosomal dominant hypophosphatemic rickets (ADHR) [6, 9], and autosomal recessive hypophosphatemic rickets 1 and 2 (ARHR1 [10, 11] and ARHR2 [12, 13]) are caused by mutations in PHEX, FGF23, dentin matrix protein 1 (DMP1), and ectonucleotide pyrophosphatase/phosphodiesterase 1 (ENPP1) genes, respectively.