LEP and obesity due to melanocortin 4 receptor deficiency: Genetic defects disrupting the leptin-melanocortin signaling pathway very often result in severe early-onset obesity and hyperphagia [4–6], and the genes most commonly involved in monogenic forms of obesity are part of this pathway, including leptin (LEP), the leptin receptor (LEPR), and the melanocortin 4 receptor (MC4R) [7–9].