Background . Fibroblast growth factor-23 (FGF23), a circulating protein produced in bone, causes decreased renal inorganic phosphate (Pi) reabsorption by reducing the expression of the sodium phosphate cotransporter type 2a (Npt2a). We have previously generated transgenic mice expressing human wild-type (WT) FGF23 under the control of the α1 (I) collagen promoter.