Interestingly, body weights and fat mass of Cpt1b+/− mice were still lower than that of WT littermates, despite the presence of a severe insulin resistant state, suggesting that specific factors (rather than obesity per se) in skeletal muscle may cause the impaired insulin sensitivity in Cpt1b+/− mice after prolonged exposure to the HFD feeding. The gene discussed is INS; the disease is obesity due to melanocortin 4 receptor deficiency.