The most severe form of obesity in mice and humans results from a deficiency in the genes encoding leptin (LEP) or the leptin-receptor (LEPR) (Friedman and Halaas, 1998; Zhang et al., 1994; Chen et al., 1996; Montague et al., 1997). This evidence concerns the gene LEPR and obesity due to melanocortin 4 receptor deficiency.