XLH is an inherited disease of phosphate metabolism in which inactivating mutations of the Phosphate-Regulating Endopeptidase Homolog, X-Linked (PHEX) gene lead to local and systemic effects including hypophosphatemia, bone defects such as rickets and osteomalacia, and elevated levels of FGF23. The gene discussed is FGF23; the disease is osteomalacia.