ATM and neoplasm: IHC quantification of γH2AX percentage of positive nuclei in the tumours, fixed 6 h after the final dose, revealed an increase in the ATRi/gem groups compared to vehicle; mean γH2AX positivity increased from 5.7 to 11.4% in WT tumours with ATRi/gem (p < 0.01) and from 7.3 to 15.4% in ATM-KO tumours with ATRi/gem compared to vehicle (p < 0.0001) (Figs. 6d and S5D).