Bcl-XL inhibits membrane permeabilization by competing with Bax.

Although Bcl-XL and Bax are structurally similar, activated Bax forms large oligomers that permeabilize the outer mitochondrial membrane, thereby committing cells to apoptosis, whereas Bcl-XL inhibits this process. Two different models of Bcl-XL function have been proposed. In one, Bcl-XL binds to an activator, thereby preventing Bax activation.

Development of dimeric modulators for anti-apoptotic Bcl-2 proteins.

Bcl-2 family proteins can be classified into two subfamilies--anti-apoptotic members and pro-apoptotic members. Mechanistically, these two subfamilies can antagonize each other through heterodimerization while homodimerization has been proposed for each subfamily to carry out their corresponding anti-apoptotic or pro-apoptotic functions. To date, many small-molecule antagonists against anti-apoptotic Bcl-2 proteins have been developed, which are monomeric modulators.