Leptin levels are increased in obesity in proportion to fat mass and leptin is well-known to act in the central nervous system to reduce food intake by regulating neuropeptides in the hypothalamus; for example by reducing the activity of neurons containing neuropeptide-Y (NPY) and agouti-related protein (AgRP) and stimulating pro-opiomelanocortin (POMC)/cocaine and amphetamine regulated transcript (CART) neurons in the arcuate nucleus of the hypothalamus (Elias et al., 1998). The gene discussed is LEP; the disease is obesity due to melanocortin 4 receptor deficiency.