Differential mRNA expression of candidate reference genes in matched non-malignant (N; blank circles) and malignant renal cell carcinoma (ccRCC; black circles) tissue samples shown as connected lines. (a-c). Gene expression of non-regulated reference genes PPIA and TBP. The Normalization Factor was calculated from both non-regulated genes with the program geNorm (see text). (d-g). Gene expression of up-regulated candidate reference genes GAPDH, ACTB, TUBB, and RPLPO. (h-k). Gene expression of down-regulated candidate reference genes HPRT1, ALAS1, HMBS, and SDHA. Significances were calculated with the Wilcoxon test of paired samples.