Because the obese phenotype of adults with PWS markedly differs from that of BMI-matched subjects with common obesity, the present study was undertaken to explore circulating levels of irisin in two adult models of obesity in relation to body composition and metabolic profiling, so as to gain further insights on its role as a myokine or adipokine in the obese setting. This evidence concerns the gene FNDC5 and obesity due to melanocortin 4 receptor deficiency.