Fibroblast Growth Factor 23 (FGF-23) is a secreted protein that belongs to the heparin-binding growth factors family. FGF-23 is expressed in osteogenic cells, particularly during phases of active bone remodeling. FGF family members possess broad mitogenic and cell survival activities, involved in a variety of biological processes including embryonic development, cell growth, morphogenesis, tissue repair, tumor growth, and invasion. FGF-23 regulates homeostasis of phosphate and vitamin-D metabolism. FGF-23 inhibits renal tubular phosphate transport by reducing SLC34A1 levels, and negatively regulates osteoblast differentiation and matrix mineralization. FGF-23 also upregulates EGR1 expression in the presence of KL, acts directly on the parathyroid to decrease PTH secretion. Defects in FGF-23 are the cause of autosomal dominant hypophosphataemic rickets (ADHR).