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Light micrograph (LM) of Insulin crystals. Insulin is a polypeptide hormone that regulates carbohydrate metabolism. It also takes part in the metabolism of fat (triglycerides) and proteins. Patients with Type 1 diabetes mellitus depend on exogenous insulin (injected subcutaneously) for their survival because of an absolute deficiency of the hormone; patients with Type 2 diabetes mellitus have either relatively low insulin production or insulin resistance, and occasionally require insulin administration if other medications are inadequate in controlling blood glucose levels. Insulin is synthesized in humans and other mammals within the beta cells of the islets of Langerhans in the pancreas. Insulin is built from 51 amino acids and is one of the smallest proteins known. Insulin is produced as a prohormone molecule - proinsulin - that is later transformed by proteolytic action into the active hormone. Magnification: x30 when shortest axis printed at 25(KEYSTONE/SCIENCE PHOTO LIBRARY SPL/DENNIS KUNKEL MICROSCOPY/SCIENCE PHOTO LIBRARY)