PARP1 and cancer: However, other studies revealed ATM deficiency may not be enough to fully sensitize these cells to PARP inhibition; experiments showed olaparib given alone to ATM-deficient cancer cells induced only a cytostatic state, while there are emerging data suggesting that olaparib combined with an ATR (ATM- and RAD3-related) inhibitor provides an additional cytotoxic effect [65,66].