KRAS and cancer: Our gastric cancer PDCs express wild-type KRAS, while KRAS-mutated cancer cells are known to be resistant to EGFR inhibitors.21 To test whether the EGFR inhibitors still affect CD44v-positive cells in KRAS-mutated cancers, we additionally examined the effect of EGFR inhibitors in SW480, a KRAS-mutant colorectal cancer cell line that is resistant to EGFR inhibition, as determined by the DepMap (https://depmap.org/portal/).22 We found that EGFR inhibition by afatinib led to a marked decrease in the population of CD44v9-expressed cells in SW480 cells (Supplementary Fig. 2B, C).