MCL1 and B-cell chronic lymphocytic leukemia: Relative to other hematological malignancies, CLL is a non-proliferative form of leukemia associated with constitutive activation of the B-cell receptor signaling pathway and overexpression of the B-cell lymphoma 2 (BCL-2) family of anti-apoptotic proteins (e.g., BCL-2, MCL-1) (195, 196).