FNDC5 and obesity due to melanocortin 4 receptor deficiency: In an attempt to account for the health benefits induced by physical activity on metabolic status, in terms of the browning (i.e., conversion) of white adipose tissue (WAT) and its resistance to diet-induced obesity, irisin was designated a hormonal factor or myokine (a polypeptide of 112 amino acids) cleaved under PGC1-α transcriptional control from the transmembrane precursor fibronectin type III domain-containing 5 (FNDC5) [1] in response to exercise.