This was achieved in a group of 49 patients treated for metastatic colorectal cancer with a combination of 5-FU and irinotecan; the pharmacokinetics of irinotecan and its metabolites were studied in all patients; the polymorphisms of UGT1A1 and CES2 were studied as potential markers for SN-38 glucuronidation and irinotecan activation, respectively; and the potential activity of CYP3A4 was estimated from cortisol biotransformation into 6β-hydroxycortisol (Yamamoto et al, 2000). This evidence concerns the gene CYP3A4 and metastatic colorectal cancer.