The ELISA study showed that irisin concentrations were lower and less variable in obese PWS subjects that controls with common obesity (Fig. 2) and the exclusion of 3 outliers from the group with common obesity (irisin concentrations, >40 ng/ml), left the difference between PWS and controls with common obesity unchanged (mean values 23.2 ± 1.2 ng/ml, p < 0.05). This evidence concerns the gene FNDC5 and obesity due to melanocortin 4 receptor deficiency.