The importance of genetic variation in clinical response to various drugs is now well recognized. Identification of genetic biomarkers that can predict efficacy and toxicity of chemotherapeutic drugs in cancer patients holds great promise in treatment improvement and cost reduction. Mitomycin C (MMC) is a common anticancer drug used for the treatment of numerous types of tumors. Metabolismmediated activation, by either one-electron or two-electron reduction, plays a critical role in the chemotherapeutic action of MMC. NADPH-cytochrome P450 (oxido)reductase (POR) is a major enzyme responsible for MMC activation through the one-electron reductive pathway, which leads to the production of semiquinone anion radicals and subsequent DNA damage in the cells. Recently, a total of six naturally occurring human POR variants with single amino acid changes (Y181D, A287P, R457H, V492E, C569Y, and V608F) have been identified. Although the catalytic efficiency of these variants in reduction of cytochrome c was reported to be altered, their capability in activating MMC, a direct substrate of POR, has not been examined. In the present study, we demonstrated that except for the C569Y variant, MMC-induced toxicity assayed as cell viability and proliferative capability was significantly decreased in the Flp-In Chinese hamster ovary cells stably expressing all the other POR variants in comparison with the cells expressing wild-type human POR. Cells expressing the V608F and Y181D variants had a complete loss of the capability to activate MMC. Our finding suggests that these functional POR genetic variations may serve as a potential biomarker to predict the chemotherapeutic response to MMC.