In a typical culture environment, Caco-2 cells, which are derived from human colorectal adenocarcinoma and are being used worldwide as the gold standard of in vitro intestinal models, do not express substantial amounts of CYP3A4 (56), CYP2C9 (57), or CYP1A and express a small amount of CYP2D6 (58). Here, CYP2D6 is linked to colorectal adenocarcinoma.