TYMS and colorectal carcinoma: The main anticancer mechanismof 5-FU is inhibiting TS by FdUMP, an active metabolite of 5-FU.1,2,17 The fundamental mechanism forthis activity, proposed by Santi in 1980,4 is that FdUMP forms a covalent ternary complex with TS and CH2-THF.4 We have investigated the mechanisms of resistanceto 5-FU in human CRC cell models, 5-FU-resistant HCT116RF10 cells, and parental HCT116 cells, revealing their genetic backgroundby exome analysis.