B-cell lymphoma-extra large (Bcl-xl, or BCL2-like 1 isoform 1) is a transmembrane molecule in the mitochondria. It is a member of the Bcl2 family of proteins, and acts as a pro-survival protein by preventing the release of mitochondrial contents such as cytochrome c, which would lead to caspase activation. It is a well-established concept in the field of apoptosis that relative amounts of pro- and anti-survival Bcl-2 family of proteins define whether the cell will undergo cell death: if more Bcl-xL is present, then pores are non-permeable and the cell survives. However, if Bax and Bak become activated, and Bcl-xL is sequestered away by gatekeeper BH3-only factors (e.g. Bim), causing a pore to form, cytochrome c is released leading to initiation of caspase cascade leading to apoptotic events.