PHEX and hypophosphatemic rickets: In addition, inactivating mutations in genes such as dentin matrix acidic phosphoprotein 1 (Dmp1) [8], Phosphate regulating endopeptidase homolog, X-linked (Phex) [9], or Ecto-nucleotide pyrophosphatase/phosphodiesterase 1 (Enpp1) [10], [11], have been shown to increase skeletal secretion of FGF23, resulting in hypophosphatemic rickets.