Fibroblast Growth Factor 21 (FGF21) is a protein hormoneof the liver that helps the body adapt to metabolic stresses(hunger, cold, overeating and obesity) (Fisher et al., 2010).Exogenous FGF21 reduces body weight, normalizes the lipidprofile, and increases insulin sensitivity in various models ofobesity and insulin resistance (Zhang, Li, 2014). The gene discussed is INS; the disease is obesity due to melanocortin 4 receptor deficiency.