 Topoisomerase inhibitors are frequently used as chemotherapy for different cancers, though they cause bone marrow suppression, are cardiotoxic and have variable effectiveness.A 2006 retrospective study, with long clinical follow-up, was made of colon cancer patients treated with the topoisomerase inhibitor irinotecan.In this study, 45 patients had hypermethylated "WRN" gene promoters and 43 patients had unmethylated "WRN" gene promoters.