An orally bioavailable, synthetic small-molecule antagonist of a subset of the B-cell leukemia 2 (Bcl-2) family of proteins with potential antineoplastic activity. BcI-2 family protein inhibitor ABT-263 selectively binds to apoptosis suppressor proteins Bcl-2, Bcl-XL, and Bcl-w and prevents their binding to the apoptotic effectors Bax and Bak proteins, which may trigger apoptosis in tumor cells overexpressing Bcl-2, Bcl-XL, and Bcl-w. Bcl-2, Bcl-XL, and Bcl-w are frequently overexpressed in a wide variety of cancers, including those of the lymphatic system, breast, lung, prostate, and colon, and have been linked to tumor drug resistance.