Navitoclax, a small molecule BH3 mimetic, was designed to function as a dual inhibitor against BCL-2 and BCL-xL in refractory CLL.11, 16, 24 BCL-xL inhibition in clinical settings led to dose-dependent thrombocytopenia that prompted redesigning of navitoclax into a BCL-2-specific inhibitor, ABT-199. This evidence concerns the gene BCL2L1 and B-cell chronic lymphocytic leukemia.