Insulin (INS) is a peptide hormone that plays a vital role in the regulation of glucose levels in the blood. Insulin in produced as a precursor (proinsulin) which is then cleaved into three peptides: the A and B chains, which are joined by disulfide bonds to become insulin, and C-peptide. Binding of insulin to its receptor (INSR) stimulates glucose uptake. Insulin is secreted in response to glucose, and a variety of hormones such as melatonin, estrogen, leptin, growth hormone and glucagon like peptide-1. Expression is specific to the beta cells of the islets of Langerhans in the pancreas. References: Curr Diabetes Rev 2013 9(1):25