PPIA and nonpapillary renal cell carcinoma: We confirmed by multiple analysis that beta-Actin is not appropriate as internal reference for studies of gene expression in ccRCC but found that combination of two HKGs, PPIA (coding for peptidylprolyl isomerase A) and RPS13 (coding for the ribosomal protein S13), minimizes fluctuations when comparing ccRCC samples to their adjacent healthy tissues.