Administration of fibroblast growth factor-21 (FGF21) and analogues and antibodies that exhibit agonism of the fibroblast growth factor receptor 1 (FGFR1)–Klotho beta (KLB) receptor complex can reduce body weight, liver fat, insulin resistance and blood glucose in animal models of obesity and diabetes, and initial clinical studies have encouraged further interest [77]. The gene discussed is FGF21; the disease is obesity due to melanocortin 4 receptor deficiency.