High expression of thymidylate synthase in colon tumors, a marker for 5-fluorouracil resistance. Representative example of a colon tumor with a high expression of thymidylate synthase (TS), the target for 5FU. Tumors with a high TS expression generally show resistance to 5FU, those with a low expression are likely to be sensitive. Picture taken from Van Triest et al., Clin Cancer Res 2000;6;1063–70. (Fig. 2 Immunohistochemical staining of colorectal cancer samples for thymidylate synthase (TS). ×165 magnification). For details, see the Heidelberger Symposium papers in this issue.