We do note that the potential for ATR inhibition to alter the genomic stability of ARID1A mutant tumours in a genotype specific manner (and therefore potentially tumour cell specific manner) might have beneficial therapeutic effects but could also have deleterious effects in driving tumourigenesis or drug resistance by the generation of new oncogenic mutations and chromosomal rearrangements. The gene discussed is ARID1A; the disease is neoplasm.