ARID1A and neoplasm: To assess whether constitutive loss of ARID1A function, as would occur in tumours, also resulted in ATRi sensitivity, weused human colorectal HCT116 isogenic cells containing either wild-type ARID1A (ARID1A+/+) or homozygous loss-of-function ARID1A mutations (p.Q456*/p.Q456*; referred to as ARID1A−/−; Fig. 2b).