For example, mutations in SIM1, leptin receptor (LEPR), pro-opiomelanocortin (POMC), melanocortin 4 receptor (MC4R), and more recently, POU3F2 have all been associated with severe obesity, suggesting a convergence on the leptin-melanocortin pathway in association with oxytocin [25,27]. Here, SIM1 is linked to obesity due to melanocortin 4 receptor deficiency.