Bcl-x is a member of the Bcl-2 family of proteins that regulates outer mitochondrial membrane permeability. 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 Bcl-2 family. Bcl-xL is an anti-apoptotic protein that prevents release of Cytochromec from the mitochondrial intermembrane space into the cytosol. Bcl-xs is a pro-apoptotic member that can initiate programmed cell death.
