We have cloned the mouse bcl-2 (mbcl-2) genomic locus and analyzed it in detail. The gene is comprised of two exons separated by more than 15 kb. Two species of mRNAs are produced, and DNA sequencing analysis shows that they code for two proteins differing at their C terminus: a 7.5 kb transcript codes for a polypeptide of 236 amino acids, mbcl-2 alpha, and a 2.4 kb transcript, which derives from the 5' exon only, codes for a protein of 199 amino acids, mbcl-2 beta. The gene is characterized by very long (5' about 1.4 kb, and 3' about 5.1 kb) untranslated regions surrounding the relatively short coding region. We have mapped the 5' end of the mbcl-2 mRNAs by S1 protection analysis, and we have analyzed the promoter region. The expression of the mbcl-2 gene was analyzed in different cell lines and in mouse tissues. Expression is tissue-specific in adult tissues: spleen and thymus express the highest level of mbcl-2 transcripts. The mbcl-2 gene maps to mouse chromosome 1.