Capecitabine is a potent antimetabolite that is the currently accepted adjuvant treatment for colorectal cancer. As well, capecitabine is used to treat head and neck cancers, breast cancer and gastric cancer. In 1985, Takeishi et al demonstrated that thymidylate synthase's gene had a satellite in the 5' untranslated region, which consisted of 3 tandem repeats of a 28 base pair sequence. Horie et al demonstrated that these satellites were polymorphic in length due to different numbers of tandem repeats, with 2 length polymorphism existing 2 tandem repeats of 28 base pairs (2R) and 3 tandem repeats of 28 base pairs (3R). Subsequent authors have demonstrated 4 repeats (4R), five repeats (5R) and nine repeats (9R).