MEN1 and multiple endocrine neoplasia type 1: Tumors with biallelic inactivation of MEN1 resulting in lack of a (functional) menin protein are, by definition, a neoplasia, and are seen in MEN1 syndrome cases, but also in about 20% of sporadic adenomas.20 However, on histologic grounds alone, it is not possible to distinguish between parathyroid adenoma and hyperplastic parathyroid.