The MC4R variant is involved in the regulation of appetite. It influences the expression of the melanocortin-4 receptor which is found in the brain areas associated with eating regulation. One of the alleles that can be present has been found to cause disordered eating behaviours (e.g. high levels of emotional eating), elevating the likelihood of overeating and gaining weight.
Source: 1000 Genome Project. Global averages for both sexes