Indeed, invalidation of a subset of genes has been shown to confer at least partial resistance to diet-induced obesity, e.g. the μ opioid receptor [28], the retinoblastoma protein [29], the peroxisome proliferating-activated receptor gamma [30], the very-long-chain fatty acid elongase [31] or the CB1 cannabinoid receptor [32]. This evidence concerns the gene PPARG and obesity due to melanocortin 4 receptor deficiency.