Ailanen et al. found that the functional polymorphism of the human neuropeptide Y gene (Rs 16139) promotes the secretion of neuropeptide Y; inhibits insulin release through the pancreatic Y1 receptor; suppresses sympathetic nerve activity, energy expenditure, and heat production of brown adipose tissue; and increases the production of white fat tissue and insulin resistance, ultimately triggering hyperinsulinemia and impairing glucose tolerance. Here, NPY is linked to hyperinsulinism.