POR and urinary bladder carcinoma: 12). CYP activity is driven by electron donation from the NADPH:CYP oxidoreductase (POR) and abundance of POR determines metabolic capacity in the CYP system.13, 14 POR is one member of the diflavin oxidoreductase family (nitric oxide synthase is the other) and is not specifically a reductase for the CYPs but can donate electrons to heme oxygenase among other enzymes (reviewed in Ref. 15). POR expression in normal and neoplastic urothelium of humans remains unquantified, although a SNP was recently associated with increased bladder cancer risk.16