Determination of the Z factor for HTS. HTS conditions described in Figure 10 were used to determine the Z factor of the assay. HeLa cells were incubated with the vehicle DMSO or actinomycin D (Act D). Cells were fixed and fibrillarin was visualized by indirect immunofluorescent staining. Images were acquired and processed as for Figure 10, and the percentage of nucleoli that are smaller than 4 μm2 was measured. The values for average + STDEV, obtained for three independent experiments, are depicted. On the basis of these results a Z factor of 0.57 was calculated.