PPIA and colon adenocarcinoma: Systematic comparisons of gene sets in different types of tumors have recently led to the selection of a variety of optimal reference genes: SDHA and TBP for bladder [24], 18S for gastric and colorectal [27], PPIA and TBP for renal [25], HPRT1 for prostate [26], SFRS4 for hepatocellular carcinoma [23], or B2M for colon adenocarcinoma [28].