X-linked hypophosphatemia (XLH) is an X-linked dominant disorder characterized by growth retardation, rickets (softening and weakening of the bones), and hypophosphatemia [too little PO₄ in the blood] due to kidney defects in phosphate resorption and Vitamin D metabolism. XLH occurs in women heterozygous for a causative allele, hence its characterization as Dominant. It is however much less severe in women, as the standard allele on the alternative X chromosome provides some compensation for XLH allele. The responsible gene is Phex (Phosphate-regulating gene homologous to Endopeptidases on the X chromosome) ( Xp22.2-p22.1).