ABT-199 is a highly potent, selective, and orally bioavailable BCL-2 inhibitor. ABT-199 has picomolar affinity for BCL-2 (Ki < 0.010 nM) and > 1000 folds selectivity over BCL-XL (Ki = 48 nM) and BCL-W (Ki = 245 nM). Therefore, ABT-199 is a much improved lead compound over the original ABT-263 (navitoclax) to avoid thrombocytopenia caused by BCL-XL inhibition. ABT-199’s cell-killing effect is selective and mechanism dependent. It can potently kill BCL-2-overexpressing FL5.12 cells (EC50 ~ 4 nM) and RS4;11 BCL-2–dependent ALL cells (EC50 ~8 nM), but showed much weaker activity against BCL-XL-overexpressing FL5.12 cells (EC50 ~261 nM) and H146 ALL cells (EC50 ~4, 260 nM). ABT-199 inhibits the growth of BCL-2–dependent human hematological tumors in vivo and spares human platelets as a single agent or in combination with rituximab and bendamustine. A single dose of ABT-199 in three patients with refractory chronic lymphocytic leukemia resulted in tumor lysis within 24 h. These data indicates that selective pharmacological inhibition of BCL-2 shows promise for the treatment of BCL-2–dependent hematological cancers.