For instance, obesity seems to positively regulate the concentrations of irisin in adipocytes and muscles to respond to an uncommon metabolic condition (Park et al., 2013; Crujeiras et al., 2014; Pardo et al., 2014; Sahin-Efe et al., 2018). The gene discussed is FNDC5; the disease is obesity due to melanocortin 4 receptor deficiency.