Previously, we found in human populations that single nucleotide polymorphisms (SNP) of CYP2A6 (CYP2A5 in mice) were detected in class I obesity patients with a body mass index (BMI) of 30–34.9 but not in class II and III obesity patients with BMI > 35, implicating that CYP2A6 is associated with the early stages of obesity; correspondingly, differences in HFD-induced body weight gain between the cyp2a5−/− mice and WT mice was observed as early as 6 weeks [14]. This evidence concerns the gene CYP2A6 and obesity due to melanocortin 4 receptor deficiency.