BCL2L1 is a member of he BCL-2 protein family. BCL2L1 is expressed as two distinct isoforms,Bcl-xL and Bcl-xS,and is located at the outer mitochondrial membrane. The Bcl-XL isoform is a 233 amino acid protein,acting as an apoptotic inhibitor. Bcl-XL can forms heterodimers with BAX,BAK or BCL2,and the heterodimerization with BAX does not seem to be required for anti-apoptotic activity. The Bcl-XS isoform is a shorter variant that is 178 amino acids in length and lacks a 63 amino acid region (amino acids 126-188),acting as an apoptotic activator.