Acetylation of Sp1 precedes p21 up-regulation in response to all HDACi. Panel A shows EC50 values calculated from the concentration-response data shown in Fig 2, for each of Sp1 acetylation and p21 upregulation. There was generally close agreement between these values. The third column indicates the concentration used in subsequent timecourse experiments. Panel B shows timecourse experiments. HCT116 cells were treated with concentrations of HDACi as indicated in panel A, for times from 0 to 6 h. Timepoints less that 1 h are 10 min and 30 min. Cells were fixed and stained for acetyl-Sp1 and for p21 as before. Fluorescences were quantitated using high content approaches. Graphs are for acetyl-Sp1 (filled squares) p21 (filled triangles) and total Sp1 (filled diamonds). The results show the means of three repeat experiments with internal triplicates.