The Bcl-2 family of proteins plays a pivotal role in determining the life and death of a cell. Members of the Bcl-2 family all possess at least one of four conserved Bcl-2 homology domains (BH1-BH4) which mediate protein-protein interactions. They can be divided into molecules which have either an anti-apoptotic or pro-apoptotic function. The ratio between these two groups of molecules can help determine whether a cell lives or dies. The majority of anti-apoptotic family members contain common BH1-BH4 domains and are most similar to Bcl-2. Pro-apoptotic family members can be further divided into two subfamilies (i) Bax, which also contains Bak and Bok; these proteins have similar sequence motifs in their BH1-BH3 domains, and (ii) "BH3 only domain" proteins which include Bad, Bid, Bik/Nbk, Blk, Hrk/DP5, and NIP3. A novel "BH3 only domain" protein designated Bim (BOD) was identified from human, mouse and rat by screening a cDNA expression library with a Bcl-2 protein probe. Bim induces apoptosis and binding to Bcl-XL and Bcl-w inhibits its apoptotic activity. Bim is expressed in a number of B and T cell lines, as measured by Northern blot analysis. Bim has a C-terminal hydrophobic domain which allows Bim to be associated with cytoplasmic membranes. Further studies have shown that Bim is associated with the microtubule dynein light chain motor complex via the cytoplasmic LC8 dynein light chain. Bim dissociates from the dynein motor-complex upon receiving an apoptotic signal, allowing both Bim and LC8 to translocate to the mitochondria where Bim can counteract the effect of Bcl-2-like proteins. Bim is detected at approximately 23 kDa in western blot analysis. Polyclonal rabbit anti-Bim antibody recognizes human, mouse and rat Bim. A peptide corresponding to amino acids 22 to 40 of human and mouse Bim was used as the immunogen. This sequence differs from rat Bim by only one amino acid.