3-month old SERT−/− mice did not display obesity (Figure 2E, F) and hepatic steatosis (data not shown), but their IRS1 and AKT responses to insulin stimulation were reduced, further indicating that SERT−/− mice develop insulin resistance prior to a significant increase in the adiposity. The gene discussed is SLC6A4; the disease is obesity due to melanocortin 4 receptor deficiency.