A tumor that forms in islet cells (hormone-making cells) of the pancreas. Pancreatic endocrine tumors may be benign (not cancer) or malignant (cancer). Islet cells make several different hormones that affect body functions, including controlling the amount of glucose (sugar) in the blood and helping digest food in the stomach. Functional pancreatic endocrine tumors make extra amounts of these hormones, which can cause symptoms. Nonfunctional pancreatic endocrine tumors do not make extra amounts of hormones, but they may cause symptoms as they grow and spread.