In a mouse model of XLH (Hyp mouse), we showed that serum FGF23 concentrations are 17-fold higher than in normal mice [7] and mitogen activated protein kinase (MAPK)/extracellular signal regulated kinase 1/2 (ERK1/2) signaling is constitutively activated in the kidney. Here, FGF23 is linked to X-linked hypophosphatemia.