PHEX and rickets: In brief, mono-allelic mutations or chromosomal derangements affecting the Phosphate Regulating Endopeptidase Homolog, X-Linked (PHEX) gene on the X chromosome lead to elevated levels of the hormone fibroblast growth factor 23 (FGF23), resulting in renal phosphate wasting, impaired 1α-hydroxylation of 25-hydroxyvitamin D [25(OH)D] to the active hormone calcitriol (1,25-dihydroxyvitamin D [1,25(OH)2D]) and consequently, chronic hypophosphatemia, impaired skeletal mineralization and rickets (9).