In the clinical setting, FGF21 levels are elevated in a range of human diseases, including obesity [15,16,17,18], non-alcoholic fatty liver disease [18,19,20,21], diabetes mellitus [22], chronic kidney disease [23,24], atherosclerosis [25,26], and coronary heart disease [27,28]. Here, FGF21 is linked to obesity due to melanocortin 4 receptor deficiency.