Fibroblast growth factor 21 (FGF21) is a member of the FGF19 subfamily that was discovered to be a critical metabolic regulator for maintenance of glucose and lipid homeostasis [1], thus emerging as a promising novel class of therapeutic for complex metabolic diseases such as type 2 diabetes (T2D) and obesity [2,3]. Here, FGF21 is linked to obesity due to melanocortin 4 receptor deficiency.