A novel human member of the Bcl-2 family Bcl-B has been identified, which is closest in amino-acid sequence homology to the Boo (Diva) protein. The Bcl-B protein is widely expressed in adult human tissues. The Bcl-B protein modulates apoptosis. Bcl-B also binds Bcl-2, BCI-XL, and Bax but not Bak. Bcl-B displays a unique pattern of selectivity for binding and regulating the function of other members of the Bcl-2 family.