The invention relates to the discovery of a novel tumor suppressor gene which is associated with multiple endocrine neoplasia type 1. The gene has been designated MEN1 and the gene product is menin. The absence of this protein and associated mutations in the corresponding gene have been identified in individuals suffering from multiple endocrine neoplasia type 1. The identification of this marker for multiple endocrine neoplasia type 1 has diagnostic uses as well as for gene therapy.