Apoptosis regulator proteins of the Bcl-2 family, named after B-cell lymphoma 2. This alignment model spans what have been described as Bcl-2 homology regions BH1, BH2, BH3, and BH4. Many members of this family have an additional C-terminal transmembrane segment. Some homologous proteins, which are not included in this model, may miss either the BH4 (Bax, Bak) or the BH2 (Bcl-X(S)) region, and some appear to only share the BH3 region (Bik, Bim, Bad, Bid, Egl-1). This family is involved in the regulation of the outer mitochondrial membrane's permeability and in promoting or preventing the release of apoptogenic factors, which in turn may trigger apoptosis by activating caspases. Bcl-2 and the closely related Bcl-X(L) are anti-apoptotic key regulators of programmed cell death. They are assumed to function via heterodimeric protein-protein interactions, binding pro-apoptotic proteins such as Bad (BCL2-antagonist of cell death), Bid, and Bim, by specifically interacting with their BH3 regions. Interfering with this heterodimeric interaction via small-molecule inhibitors may prove effective in targeting various cancers. This family also includes the Caenorhabditis elegans Bcl-2 homolog CED-9, which binds to CED-4, the C. Elegans homolog of mammalian Apaf-1. Apaf-1, however, does not seem to be inhibited by Bcl-2 directly.