X-linked hypophosphatemia [XLH; OMIM (Online Mendelian Inheritance in Man) # 307800; incidence, 3.9 of 100,000 births] (1) is the most common form of genetic rickets associated with increased levels of circulating fibroblast growth factor 23 (FGF23). Here, FGF23 is linked to X-linked hypophosphatemia.