Most notably, human obesity has been associated with mutations in leptin (LEP) [5], leptin receptor (LEPR) [6], prohormone convertase 1 (PC1) [7], proopiomelanocortin (POMC) [8] and melanocortin-4 receptor (MC4R) [9–15]. Here, PCSK1 is linked to obesity due to melanocortin 4 receptor deficiency.