We recentlyhypothesized that 5-FU-resistant CRC cells have upregulated TYMS expressionand use a fraction of their TS to trap FdUMP, resulting in 5-FU resistance.16 Indeed, the protein levels of free-TS, FdUMP-TS-CH2-THFcovalent complex, and total TS were significantly higher in HCT116RF10 cells than in HCT116 cells under the passage culture conditions(Figures 1C,D). The gene discussed is TYMS; the disease is colorectal carcinoma.