FNDC5 and obesity due to melanocortin 4 receptor deficiency: In more detailed analysis, irisin levels of PWS patients with DEL15 were significantly reduced compared with controls with common obesity (21.8 ± 0.8 ng/ml and 25.8 ± 1.1 ng/ml, p=0.018) while PWS patients with UPD15 did not show significant differences compared with controls (23.3 ± 1.0 ng/ml and 25.7 8 ± 1.1 ng/ml, p=0.2).