FGF23 (fibroblast growth factor 23) is a member of the fibroblast growth factor family and controls phosphate and vitamin D homeostasis. The full-length protein comprises 251 amino acids including a 24 amino acid signal peptide. The N-terminal FGF homology region of FGF23 is separated from the unique C-terminal region by a proteolytic cleavage site. A proportion of FG23 is proteolytically processed between arginine179 and serine180 to generate N-terminal and C-terminal fragments. Therefore, the major forms of FGF23 present in human circulation are hormonally intact FGF23 and inactive N-terminal and C-terminal fragments. FGF23 binds to FGF receptor 1c (FGFR1c) with its N-terminal region, while the C-terminal region is capable of interacting with the co-receptor αKlotho to confer high-affinity binding to the receptor. FGFR1c and αKlotho are expressed in the distal nephron and the parathyroid gland. Co-receptor independent signaling of FGF23 has been described for other FGFRs, which are expressed in a variety of tissues. The main source of FGF23 are osteocytes in the bone.