BACKGROUND:Fluorouracil is a basic chemotherapy drug for gastric cancer, and its drug resistance is commonly seen in clinic. Cancer stem cels have low sensitivity to chemotherapy drugs, which may be an important cause of tumor recurrence and progression folowing chemotherpay. OBJECTIVE: To explore the sensitivity of gastric cancer stem cels to 5-fluorouracil and related biology mechanism underlying multidrug resistance. METHODS: Stem cel marker CD44 and multidrug resistance protein thymidylate synthase in 69 cases of gastric cancer tissues were tested by immunohistochemical staining. The selection strategy was based on the clonal morphology, and gastric cancer stem cels were isolated from human gastric cancer cel line AGS to detect the expression of CD44 and thymidylate synthase and the self renewing ability. Inhibitory concentration 50 (IC50) of 5-fluorouracil in different AGS cel clone was detected using cel counting kit-8. RESULTS AND CONCLUSION:In 69 cases of gastric cancer tissues, the positive expression rates of thymidylate synthase and CD44 were 57% (39/69) and 61% (42/69), respectively, and there was a positive correlation between the expression of CD44 and thymidylate synthase (Kappa=0.41,χ2=11.59,P < 0.05). Cloning efficiency of the AGS cel line was 39% (29/69), wherein, vice cloning, secondary cloning, and ful clone proportions were 17% (5/29), 69% (20/29), 14% (4/29), respectively. After sub-cloning, the clone was digested using trysin folowed by low-density passage; vice clones were unable to passage, secondary clones could only passage a few, and ful clones could serialy passage. After treatment with different concentrations of 5-fluorouracil, the ful clone growth inhibition rate was significantly lower than that of secondary clones and AGS cels (P < 0.05). These findings show that gastric cancer stem cels are less sensitive to 5-fluorouracil, indicating the cels have a resistance to chemotherapy drugs, which may be one of the chemotherapy resistance mechanisms in the clinical treatment of gastric cancer.