Specific knockdown of BTK impairs survival in CLL cells. (A) Knockdown of BTK by siRNA. Western blot analysis of BTK protein levels 72 hours after nucleofection with control or BTK-specific siRNA shows knockdown of BTK. This is representative of 37 patient samples. (B) CLL cell viability 72 hours after nucleofection with control or BTK siRNA. Following BTK knockdown, cells were cultured for 72 hours, and viability was determined at that time by percentage of cells negative for PI and Annexin V. Cells transfected with BTK siRNA had diminished viability compared with those treated with control siRNA. (C) Viability of CLL cells after transfection with BTK or control siRNA and coincubation with stromal cells or stimulation with IgM. BTK knockout reduced CLL cell viability even in the presence of stromal cells or after IgM stimulation.