Depsipeptide inhibition of histone deacetylase activity in CLL cells. Depsipeptide inhibits histone acetylase at concentrations that promote histone acetylation in vitro in CLL cells. CLL cells were incubated in media or depsipeptide (0.000 38, 0.0038, 0.038 and 0.38 μM) for 4 hours. Histone deacetylase activity was measured by conversion of a tritiated K562 histone substrate. Histone deacetylase activity in the media control is set at 100%, and the depsipeptide treatment data are expressed relative to this. Error bars represent 95% confidence intervals (CI).