Leptin deficiency leads to obesity and concomitant hormonal changes that might exert a neurotrophic effect on brainstem circuits, e.g., increased levels of amylin and insulin (20, 32), that may perhaps compensate over time for the lack of leptin’s neurotrophic action in ob/ob while the animals gain weight. The gene discussed is LEP; the disease is obesity due to melanocortin 4 receptor deficiency.