The murine homolog of the multiple endocrine neoplasia type 1 (MEN1) gene (men1), which in humans is associated with tumors of the parathyroids, pancreas, and pituitary, has been characterized by isolating 27 clones from a mouse embryonic stem cell cDNA library. The insert sizes ranged from 600-2500 bp, and sequence analysis identified a 1833 bp open reading frame encoding a 611 amino acid protein. In addition, two clones contained an unspliced intron 1, and another two clones contained 20-29 bp of an upstream sequence, which suggested the presence of an alternate exon 1. This was supported by an analysis of the homologous human sequence. The mouse and human coding regions had 89% and 96% identity of the nucleotide and amino acid sequences, respectively. Investigation of clones isolated from a 129ola mouse genomic library, revealed the men1 gene to consist of 10 exons that spanned approximately 6 kb. Northern blot analysis demonstrated the ubiquitous expression of 2.9 kb and 3. 4 kb transcripts in mouse adult tissues and embryos from 7 days. DNA sequence analysis of the larger 3.4 kb transcript revealed it to result from a retention of intron 1. In situ hybridization confirmed an early ubiquitous expression in whole mount mouse embryos and adult tissues, but in the latter, different levels of cellular expression were observed, e.g., men1 expression was higher in testicular Sertoli cells than in germ cells. Thus, the mouse men1 gene and the basis of alternative transcripts have been defined, and these will help to facilitate studies of a mouse model.