MEN1 and tumor of parathyroid gland: A MEN1 gene mutation has been observed in patients exhibiting MEN1, an autosomal dominant genetic disease that is characterized by the development of primary tumors that involve two or more endocrine tissues within a single patient, including tumors of the parathyroid gland (95% of cases), pancreatic islets (30–80% of cases) and anterior pituitary gland (15–90% of cases).