MEN1 and gastric neuroendocrine tumor, well differentiated, low or intermediate grade: Primary hyperparathyroidism is usually the first manifestation detected and has high penetrance in individuals with MEN1, even at younger ages.[2] Pituitary tumors are present in 40% of patients with MEN1 and are usually prolactin-producing or non-functioning tumors.[3] Type II gastric neuroendocrine tumors (NETs) are associated with MEN1, often affecting both the fundus and body of the stomach.