CLL cells undergo apoptosis in vivo after the first dose of venetoclax. Increase in the percentage of CLL cells with detectable exposure of PS after a single dose of 20 mg (dashed) or 50 mg (solid) of venetoclax among 15 patients dosed with the drug for the first time. PS exposure was detected by FITC-conjugated annexin V. Paired Student t test P = .0006 for comparison between pre-venetoclax 0 hour collection and peak post-venetoclax collection (at either 6 or 24 hours post first dose).