As a key enzyme in folate metabolism, the thymidylate synthase (TS) is important for the synthesis of nucleotides. Its polymorphisms may affect the TS gene expression and the susceptibility for Fluoropyrimidine (FU)-based chemotherapies. In this study, we assessed the relationship between the TS genotypes and clinical outcome to 5-FU-based chemotherapy, and examined whether cell-free circulating DNA is applicable for these molecular analyses. We combined the variable number tandem repeat (VNTR) and single nucleotide (SNP) polymorphisms of the TS promoter and the deletion variants (1494del6) in the 3UTR with the occurrence of loss of heterozygosity (LOH) at the microsatellite markers D18S59, D18S1140 and D18S976 mapped up- and downstream to the TS locus. Cell-free blood DNA, tumor tissues and leukocytes of 51 patients with advanced colorectal cancer were used. Genotyping revealed linkage disequilibrium between TS promoter and 3UTR (p=0.03) in blood and leukocytes. Inverse associations of the response to therapy with the number of polymorphisms (p=0.05) and the 494del6 polymorphism (p
