Bcl x is a member of the Bcl 2 family of proteins, which function as regulators of apoptosis. It may provide neuroprotection against ischemic brian injury. Bcl x has two isoforms, Bcl xl (long) a 241 amino acid protein and Bcl xs (short) a 178 amino acid protein lacking a 63 amino acid domain that is well conserved among members of the Bcl2 family. Isoform Bcl-X(L) anti-apoptotic activity is inhibited by association with SIVA isoform 1. Inhibits activation of caspases. Appears to regulate cell death by blocking the voltage-dependent anion channnel (VDAC) by binding to it and preventing the release of the caspase activator, cytochrome c, from the mitochondrial membrane. The Bcl-X(S) isoform promotes apoptosis.