Sorafenib causes increased apoptosis in ZAP-70⁺ CLL cells. CLL cells were cultured in the presence of sorafenib or DMSO control, added once at the beginning of the culture. CLL cells were harvested at the indicated times and stained with DiOC₆/PI and analyzed by flow cytometry. (A) Presented are contour maps from 2 representative ZAP-70⁺ and 2 ZAP-70⁻ CLL samples treated with DMSO or 5μM sorafenib for 24 hours. The relative DiOC₆ and PI fluorescence intensities are depicted on the x- and y-axis, respectively. Cells in the lower right quadrant, which are DiOC₆ bright and PI negative, are viable, and those numbers were used for the generation of the plots shown below. (B) CLL cells were cultured in the presence of 5μM sorafenib or DMSO control and harvested after 24 and 48 hours for analysis of viability as above. Results are represented relative to untreated control at day 0, which was set as 100%. Data shown are mean ± SD from ZAP-70⁻ (n = 5) and ZAP-70⁺ CLL cells (n = 5); * indicates a statistically significant difference; P < .05 paired Student t test. (C) CLL cells were cultured in increasing doses of sorafenib and analyzed for viability as above.