MCL1 and B-cell chronic lymphocytic leukemia: CDK9 is also responsible for transcription of anti-apoptotic factors, that belong to Bcl-2 superfamily: Mcl-1 (Myeloid cell leukaemia-1), Bcl-2 (B-cell CLL/Lymphoma 2), and XIAP (X-linked inhibitor of apoptosis) [242], making CDK9 potential source of inhibition for anti-tumor purposes.