5‐FU, an antimetabolite drug widely used in the treatment of several cancers including colorectal and breast cancers, exerts its anticancer effects through inhibition of TS and incorporation of its metabolites (FdUMP, FUTP, and FdUTP) into RNA and DNA.21, 22, 23 On the other hand, TS could also be used to predict the sensitivity of tumor cells to 5‐FU as a molecular biomarker. The gene discussed is TYMS; the disease is breast carcinoma.