SIRT1 and obesity due to melanocortin 4 receptor deficiency: Mammalian genomes encode seven Sir2 homologs (SIRT1-7); while it is unclear whether overexpression or hyperactivation of any of these sirtuins can extend lifespan, there is evidence that these genes can protect against several age-related pathologies, including neurodegeneration [9], hearing loss [10], diet induced obesity [11, 12] and cancer [13-15].