As FGF21 administration in obese animal models has shown an improvement in insulin sensitivity, a decrease in triglyceride and cholesterol levels, and a reduction in adiposity, FGF21 is expected as a new therapy for obesity and obesity-related diseases, including type 2 diabetes and nonalcoholic steatohepatitis4–6. The gene discussed is INS; the disease is obesity due to melanocortin 4 receptor deficiency.