Bcl-x, a member of the Bcl-2 protein family, inhibits cell death, or apoptosis. 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. It appears to regulate cell death by blocking the voltage-dependent anion channel (VDAC) by binding to it and preventing the release of the caspase activator, CYC1, from the mitochondrial membrane. Also acts as a regulator of G2 checkpoint and progression to cytokinesis during mitosis.