X-linked hypophosphatemia is a heritable form of rickets characterized biochemically by phosphaturia and abnormal bioactivation of vitamin D. Recent advances include the observation, using kidney cells from the X-linked hypophosphatemia mouse model (Hyp), that in-vitro renal phosphate transport is normal yet bone mineralization may be intrinsically abnormal. Of special interest is the identification of a gene (PEX) that is mutated in X-linked hypophosphatemic patients.