ATM Kinase Inhibitor (KU 55933) is a cell-permeable, potent, selective and ATP-competitive inhibitor of ATM (Ataxia telangiectasia mutated), a serine/threonine protein kinase, that exhibits an IC50 of 13 nmol/L and a Ki of 2.2 nmol/L. ATM is rapidly and specifically activated in response to DNA double-strand breaks in eukaryotic cells and signals to cell cycle and DNA repair components by phosphorylating downstream targets such as p53, CHK2, NALP2 (NBS1), and BRCA1.