BCR-stimulated CLL cells are resistant to ABT-199–mediated apoptosis. (A) CLL cells (n = 47) were cultured for 24 hours with or without ABT-199 or ABT-199 + immobilized anti-IgM (imm-aIgM). ABT-199 (2 nM) was added 3 hours after imm-aIgM. The percentage of viable cells was determined by annexin V/PI staining and flow cytometry. Significance of differences in leukemic cell viability was evaluated with the paired Student t test. (B) One representative experiment is shown. Viable cells bound with beads coated with imm-aIgM are seen as a separate annexin V–negative/PI-negative population.