LEP and obesity due to melanocortin 4 receptor deficiency: Moreover, Turnbaugh et al. demonstrated that genetically obese mice (i.e., homozygous for a mutation in the leptin gene (ob/ob) that produces a stereotyped, fully penetrant obesity phenotype) compared to their lean siblings (ob/+ and +/+) have a gut microbiome composition that promotes obesity through excess SCFA production and increased energy availability [14].