Fibroblast growth factor 23 (FGF23) is is an endocrine member of the family of FGFs and mainly produced in the bone and, upon secretion, forms a complex with a FGF receptor and coreceptor αKlotho. FGF23 can exert several endocrine functions, such as acting as a hormone on the kidney, stimulating phosphate excretion and suppressing formation of 1,25(OH)2D3, active vitamin D. Moreover, it has paracrine activities on several cell types, including neutrophils and hepatocytes. FGF23 and phosphate have been revealed to be factors relevant in cancer. FGF23 is particularly significant for those forms of cancer primarily affecting bone (e.g., multiple myeloma) or characterized by bone metastasis.