Treatment of T2D patients using leptin replacement has been reported to improve their metabolic disorder status, to reduce insulin insensitivity and reverses obesity (Farooqi et al., 2002; Gibson et al., 2004; Licinio et al., 2004; Paz-Filho et al., 2008), suggesting that leptin dysfunction may also directly affect glucose metabolism (Margetic et al., 2002). The gene discussed is LEP; the disease is type 2 diabetes mellitus.