Bcl-x, a member of the Bcl-2 protein family, inhibits cell death, or apoptosis (1). Bcl-x is expressed as two isomeric forms, Bcl-xL and Bcl-xS, and is typically present in the cytosol in association with the mitochondrial membrane. Bcl-xL forms heterodimers with various proteins, including Bax, Bak and Bcl-2 (2). It has been found that heterodimerization with Bax does not seem to be required for anti-apoptotic activity (3). Since Bcl-xL can form an ion channel in synthetic lipid membranes, there is a strong possibility that this property plays a role in heterodimerization-independent cell survival (4). The Bcl-X(S) isoform promotes apoptosis.
