TNF and obesity due to melanocortin 4 receptor deficiency: Subsequent animal studies with KO have included a wide range of models such as ulcerative colitis [21], depression [22], obesity [23,24], and myocardial infarction in rats [25], or tumor necrosis factor-alpha (TNFα)-overexpression [26], rheumatoid arthritis [27], and dietary obesity in mice [28,29,30,31].