MEMBERS Of the Bcl-2 family of proteins are characterized by their ability to modulate cell death, Bcl-2 and some of its homologues inhibit apoptosis(1-4), whereas other family members, such as Bax, will accelerate apoptosis under certain conditions(5). Here we describe the identification and characterization of a complementary DNA that encodes a previously unknown Bcl-2 homologue designated Bak, Like Bax, the bak gene product primarily enhances apoptotic cell death following an appropriate stimulus, Unlike Bar, however, Bak can inhibit cell death in an Epstein-Barr-virus-transformed cell line. The widespread tissue distribution of Bak messenger RNA, including those containing long-lived, terminally differentiated cell types, suggests that cell-death-inducing activity is broadly distributed, and that tissue-specific modulation of apoptosis is controlled primarily by regulation of molecules that inhibit apoptosis.