["Fluoropyrimidines such as 5-fluorouracil (5-FU) and 5-fluoro-2'deoxyuridine (FUDR) are among the most effective chemotherapeutic agents for treatment of metastatic colorectal cancer (CRC). Increased expression of thymidylate synthetase (TS) in CRC metastases has been proposed to be an important mechanism of resistance to fluoropyrimidine-based chemotherapy.", 'The present study investigated whether TS mRNA levels in liver metastases of 20 CRC patients before treatment with FUDR by hepatic arterial infusion (HAI) correlated with frequency of clinical response or survival duration.', 'Median survival duration of patients with TS mRNA levels above and below the median was 15 and 18 months, respectively (p > 0.05). Clinical response was achieved in 40% of patients with low TS mRNA levels, but in only 20% of patients with high TS mRNA levels (p = 0.01). TS mRNA levels were also measured for liver metastases of 7 of the patients that did not achieve a clinical response. A statistically significant increase in expression of TS mRNA was observed for liver metastases resistant to chemotherapy (21 +/- 14) in comparison to liver metastases of the same patients before chemotherapy (8 +/- 4) (p = 0.03).'] Are thymidylate synthetase mRNA levels increased in liver metastases of colorectal cancer patients resistant to fluoropyrimidine-based chemotherapy?