Moreno et al. found that irisin was produced in human preadipocytes and adipocytes and 3T3-L1 adipocytes, and irisin expression in human muscle was 200 times higher than that in adipose tissue; they also concluded that circulating irisin levels were negatively associated with obesity and insulin resistance [20]. Here, FNDC5 is linked to obesity due to melanocortin 4 receptor deficiency.