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. OR A drug used to treat chronic lymphocytic leukemia (CLL) in patients who have already received other treatment. It is used in patients whose cancer has a change in chromosome 17. It is also being studied in the treatment of other types of cancer. Venetoclax binds to a protein called BCL2, which may be found on some types of leukemia cells. Blocking this protein may help kill cancer cells and may make them more sensitive to other anticancer drugs. Venetoclax is a type of BCL2 inhibitor. Also called Venclexta.