Multiple endocrine neoplasia type 1 (MEN 1) is a relatively uncommon inherited disease that is due to mutations in MEN1 gene. Individuals who inherit the mutations for MEN 1 gene have an increased chance of developing over-activity and enlargement of certain endocrine glands. The endocrine glands most commonly affected by MEN 1 are the parathyroid glands, the pancreas, and the pituitary glands.  Almost everyone who inherits MEN 1 develops over-activity of the parathyroid glands (hyperparathyroidism) at some stage in their life. The other endocrine glands become over-active less frequently; however, people who inherit MEN 1 will usually develop over-activity in more than one endocrine gland. Over-activity in different endocrine glands may occur simultaneously or at separate times during a person's life.