To confirm drug tolerance and ALDH1A3 expression in residual gastric cancer cells after anticancer drug treatment, we collected DTP cells after treating the gastric cancer PDC lines JSC15-3 and JSC18-1 with 5-FU or CDDP for 9 days and compared their drug sensitivities with those of the parental cells. The gene discussed is ALDH1A3; the disease is gastric cancer.