Fibroblast growth factor 21 (FGF-21) is a member of the FGF family, which controls several physiological and pathological development processes, including cell growth, cell survival, and morphogenesis. FGF-21 is expressed primarily in the liver and brown adipose tissue, and is involved in the control of fat metabolism, such as activation of glucose uptake by adipocytes. In the presence of the cofactor Beta-Klotho, FGF-21 binds both receptors FGFR1c and FGFR4. Altered levels of FGF-21 in patients suffering from type 2 diabetes, liver diseases, and obesity indicate a role of this protein in the pathogenesis of several metabolic diseases. Recombinant human FGF-21 shares 81% sequence identity with mouse FGF-21 and is known to be active on mouse cells.