BCL-XL (B-cell lymphoma-extra large), an anti-apoptotic member of the BCL-2 family proteins, is prominently found in the inner membrane of mitochondria and characterized by containing all four BCL-2 homology (BH) domains (BH1 to BH4). A hydrophobic cleft (the BH3-binding groove) formed by apposition of the BH1, BH2 and BH3 domains has been identified in the molecular surface of BCL-XL, through which it interacts with pro-apoptotic BCL-2 proteins, Bax and Bak, to inhibit apoptosis. Moreover, BCL-XL also exhibits anti-autophagy abilities to avoid autophagic cell death by binding to protein Beclin 1, which is required to initiate the formation of autophagasome in autophagy.