Paraffin-embedded tissues from 54 patients with unresectable metastases from colorectal cancer who had undergone chemotherapy with the FOLFOX regimen were analyzed for MTHFR polymorphisms in the MTHFR gene (677C-->T, Ala-->Val mutation) and XRCC1 gene (Arg-->Gln substitution in exon 10).
Thymidylate synthase activity measured in metastases was a significant predictor of 5FU responsiveness and the addition of the 677C>T genotype improved model prediction.