FGF-21 (Fibroblast Growth Factor 21) is a member of an FGF subfamily that also includes FGF-19 and FGF-23. FGF-19 subfamily members lack a heparin binding domain, enabling them to freely circulate as endocrine factors and diffuse within tissues. FGF-21 signals through Klotho beta in complex with FGF receptors. It regulates cellular metabolism including glucose uptake in adipocytes and cellular sensitivity to insulin. It is basally expressed in the pancreas, thymus, liver, and adipose tissue and is upregulated in liver, muscle, and fat in obesity and diabetes. FGF-21 also plays a role in cell survival and proliferation, mesenchymal stem cell differentiation, circadian rhythm, and controlling reproductive capacity.