This is also the case in the design of clinical trials, in which drug combinations are increasingly being used, and particularly so in the treatment of cancer, where the numerous possible drug combinations involving targeted antitumor agents [most of which are P450 substrates and as shown here also PXR agonists (Zanger and Schwab, 2013)] are being tested to increase efficacy and delay the onset of drug resistance. This evidence concerns the gene CYP2B6 and cancer.