Background/Aim: This study aimed to elucidate the detailed characteristics of CYP3A5 expression and the association between CYP3A5 expression and clinical outcomes in patients with renal cell carcinoma (RCC).Patients and Methods: This study retrospectively enrolled 124 Japanese patients with RCC treated at the Okayama University Hospital. The commonest CYP3A5 gene polymorphism, CYP3A5*3, and expression levels of CYP3A5 mRNA and protein in each tissue were examined. Results: Expression of CYP3A5 mRNA and protein in RCC tissues was significantly down-regulated compared to that in adjacent normal tissues. High level of CYP3A5 mRNA expression significantly extended cancer-specific survival (p=0.004) and overall survival (p=0.002). The CYP3A5 mRNA expression level was identified as a significant independent prognostic factor for both cancer-specific survival and overall survival. Conclusion: CYP3A5 could serve as a potential marker for prognostication and treatment planning for patients with RCC.Renal cell carcinoma (RCC) is the commonest cancer of the kidney and comprises approximately 90% of all cases of kidney cancer (1). RCC is estimated to account for 2% of new cancer cases or cancer deaths worldwide (2, 3). Advances in cancer therapy have improved the 5-year survival rates for RCC, although the overall prognosis for RCC remains unsatisfactory, at approximately 65% of the current 5-year survival rates for RCC (4). Moreover, the global incidence of RCC has been gradually increasing (5).Cytochrome P450 (CYP) 3A is the most abundant CYP subfamily that is responsible for the metabolism of a large number of substrates in humans. The major CYP3A isoform is CYP3A4; however, three minor isoforms, CYP3A5, CYP3A7, and CYP3A43, have been reported (6). Of these, CYP3A5 is the most abundantly expressed enzyme of the minor CYP3A isoforms in adults (6). The CYP3A5 gene is highly polymorphic, which causes individual variations in the expression of CYP3A5. The commonest allele responsible for the variable protein expression of CYP3A5 is CYP3A5*3 (rs776746, 6986 A>G) (7). Individuals who are homozygous for the CYP3A5*3 allele either express very low levels of CYP3A5 protein or lack CYP3A5 protein expression (7,8). Interestingly, evidence from small cohort studies indicate that CYP3A5, but not CYP3A4, is markedly expressed in the normal kidney with a CYP3A5*1 allele (9, 10). Thus, CYP3A5 is considered to play a role in substrate metabolism in the kidney.Most studies on the role of CYP3A5 in cancer have focused on the metabolism of exogenous agents, such as anticancer drugs, in the liver and small intestine. Many anticancer drugs are reported to be candidate substrates of CYP3A5 (11,12). In addition, evidence has shown that CYP3A5 has several roles in cancer progression, via the metabolism of endogenous or carcinogenic substrates, that are independent of the metabolism of anticancer drugs (13).