Normalization against geometric means. Venn diagram demonstrating differences in the populations of mRNAs identified as "regulated" when the Ct values from the PCR arrays were normalized against PPIA alone, against the geometric mean of PPIA and DAD1 ("Geom Mean 2") or the geometric mean of PPIA, DAD1 and HMGB1 ("Geom Mean 3"). The analysis is based on the same 85 targets as in Figure 3A. The intersecting lines define shared and distinct target gene populations. The values refer to the number of genes apparently regulated significantly (fold change ≥ 1.5, p < 0.05) in the time course. The three methods identified the following numbers of significantly regulated genes: PPIA alone, 60; Geom Mean 2, 64; Geom Mean 3, 63.