An ATM kinase inhibitor; inhibits ionizing radiation-induced ATM autophosphorylation and phosphorylation of ATM targets in HeLa cells at 10 μM; selective for ATM over PI3K and PIKK in HFF(hTERT) and A-T(hTERT) fibroblasts, respectively; decreases cell survival in response to ionizing radiation in HeLa cells; cytotoxic to MCF-7 and SKBr-3 breast cancer cells (IC50s = 16.92 and 12.78 μM, respectively)