Apoptosis or programmed cell death is an essential physiological process required for normal development and maintenance of tissue homeostasis. However, apoptosis is also involved in a wide range of pathologic conditions, including acute neurological injuries, neurodegenerative diseases, cardiovascular diseases, immunological diseases, AIDS, and cancer. Several signaling pathways are involved in inducing apoptosis, depending on the initiating stimulus. Proteins of the Bcl-2 family together with mitochondria, cytochrome c, and caspases have among others been identified as essential components of the intracellular apoptotic signaling pathways.

Bcl-2 was first identified as a proto-oncogene in follicular B-cell lymphoma. In the lymphoma cells the Bcl-2 gene was found at the breakpoint of the translocation between chromosome 18 and chromosome 14, where the gene is under the control of the immunoglobuline heavy chain intron enhancer [1]. This results in a transcriptional upregulation of the gene and overexpression of the Bcl-2 protein. Bcl-2 was subsequently identified as a mammalian homologue to the apoptosis repressor ced-9 in C. elegans. Bax (Bcl-2 associated protein X) was the first pro-apoptotic member of the family to be identified as a protein coimmunoprecipitating with Bcl-2. The Bcl-2 family members can be subdivided into two groups according to their function, the antiapoptotic members on one side and the proapoptotic members on the other side (Fig. 1).