The Bcl-2 family consists of anti-apoptotic (or cell survival) genes, such as Bcl-2, Bcl-XL and pro- apoptotic (or cell death) genes including Bax and BAD. In addition to its anti-apoptotic effect, Bcl-2 also inhibits progression through the cell cycle. Functional interactions between family members as well as binding to other cellular proteins modulate their activities. Bcl-2 family members are important in normal tissue development, homeostasis, and disease states. Bcl-2 prevents cell death by forming heterodimers with Bax and Bad (1). Bax promotes cell death by forming homodimers, which can be blocked by forming heterodimers with Bcl-2 or BCL-XL. Recently, using yeast two-hybrid system Cuddeback et al (product citation 2) have identified a cDNA encoding a 365 amino acid protein, which interacts physically with Bax protein and influences cell life and death. Bif-1 contains a Src homology 3 (SH3) domain in the C-terminus. It does not contain any of the conserved Bcl-2 homology (BH) domains of the Bcl-2 family proteins. Human Bif-1 protein shares 96% and 42% identity at the amino acids level with mouse and C. elegans, respectively. Overexpression of Bif-1 in transfected cell lines promotes Bax conformational change, caspase activation, and apoptotic cell death following growth factor withdrawal. Bif-1 RNA is abundantly expressed in heart, skeletal muscle, kidney, and placenta.