An orally bioavailable, selective small molecule inhibitor of the anti-apoptotic protein Bcl-2, with potential antineoplastic activity. Venetoclax mimics BH3-only proteins, the native ligands of Bcl-2 and apoptosis activators, by binding to the hydrophobic groove of Bcl-2 proteins thereby repressing Bcl-2 activity and restoring apoptotic processes in tumor cells. Bcl-2 protein is overexpressed in some cancers and plays an important role in the regulation of apoptosis; its expression is associated with increased drug resistance and tumor cell survival. Compared to the Bcl-2 inhibitor navitoclax, this agent does not inhibit bcl-XL and does not cause bcl-XL-mediated thrombocytopenia.