MEN1 and multiple endocrine neoplasia type 1: Consistent with Knudson's “two-hit” hypothesis,113 the autosomal dominant condition is characterized by an acquired germline mutation in one MEN1 allele, followed by loss of the second allele within the tumor by deletion (loss of heterozygosity) or inactivating point mutations.114 Patients presenting with the MEN1 syndrome experience a higher risk for developing multiple endocrine tumors in the pancreas, pituitary, and upper GI tract.