The level of ATRi sensitivity in YCC6 and SNU5 cells was comparable with that seen in previously validated ARID1A mutant HCT116 isogenic cell pair (Fig. 1A–C; ref. 52), allowing us to select YCC6 tumor cells as a “ATRi sensitive” cell line for use in a later CRISPR-Cas9 mutagenesis screen for determinants of ATRi resistance. This evidence concerns the gene ARID1A and neoplasm.