Abnormally high FGF21 levels are considered a marker of disturbed metabolism in non-HIV-infected patients with obesity, diabetes, or congenital lipodystrophy (15, 16, 42), whereas KLB repression is associated with an impairment of glucose uptake and other health effects mediated by FGF21 (27). Here, FGF21 is linked to obesity due to melanocortin 4 receptor deficiency.