Genetic factors influencing obesity in children and adolescents are classified into three groups: (1) syndromic forms, characterized by alterations in regions in chromosomes, causing a cluster of clinical phenotypes (a syndrome); (2) monogenic forms, caused by rare mutations in a single gene with large effects (i.e., leptin, melanocortin receptor 4, among others); and (3) common obesity, the most frequent form of obesity, which is polygenic, that is, with contribution of numerous genetic variants with small effects [9]. The gene discussed is LEP; the disease is obesity due to melanocortin 4 receptor deficiency.