Expression levels of target genes in normal and tumor tissues upon different normalization approaches. Median relative quantities of target genes (TG1–12) in non-neoplastic white matter (black bars) and glioblastoma (gray bars) samples were plotted after normalization under the indicated conditions: with geNorm normalization factors calculated from TBP plus HPRT1 and with the genes GUSB or 18S rRNA alone. Whiskers indicate the standard deviation. Significance between differences was calculated by the use of Mann-Whitney test. Asterisks indicate the significance of differences, * P values < 0.05 and ** P values < 0,005. Graphs were plotted with Sigma Plot 10.0 software.