Bcl-XL is a transmembrane protein located in the mitochondrial membranes of cells that are long-lived and postmitotic， such as adult brain cells. It plays arole in the signal transduction pathway of the FAS-Ligand. Bcl-XL is an anti-apoptotic protein which is a member of the Bcl-2 family which are able to form heterodimers， and this is an significant event in the regulation of apoptosis. BCL-XL is involved in the survival of cancer cells.
Bcl-xL is the leading monitor of apoptosis/active cell suicide. Bcl-xL has cell death repressor activity and therefore acts as a survival protein.
