The current invention embodies a mouse model which is heterozygous for a null allele at the Men1 locus of murine chromosome 19. Men1 has similar exon-intron organization and amino acid identity compared with its human analog MEN1, which has been implicated in the pathogenesis of multiple endocrine neoplasia, type I (MENI). This mouse model has been shown to develop features remarkably similar to those of MEN1, which include tumors of the endocrine pancreas, pituitary, and parathyroids. The model embodied in this invention appears to represent a valuable research tool for use in elucidating the role of the wild-type Men1 allele in tumor formation, and ultimately should aid in the testing of possible therapeutic approaches to human MEN1.