Detection of apoptosis in CLL cells as detected by annexin-V/PI assay after exposure to media or depsipeptide (0.038 or 0.38 μmol/L) for a total of 4 hours, washing, and then incubation in media for 24 (A), 48 (B), 72 (C), and 96 hours (D). No apoptosis was noted with depsipeptide immediately after 4 hours of exposure (not shown). This demonstrates that, as the concentration of depsipeptide is increased, a time- and dose-dependent induction of apoptosis occurs. Phosphatidyl serine is restricted to the internal cell membrane but is externalized early during the process of apoptosis. Cellular uptake of PI indicates a disrupted cellular membrane generally observed during late apoptosis or with cell necrosis. Increased annexin binding to exposed phosphatidyl serine in quadrant 4 of (A), (B), (C), and (D) (percentages noted in far right corner) exposed to depsipeptide as compared with media supports that depsipeptide is inducing cytotoxicity at least in part through the pathway of apoptosis in human CLL cells.