ATR kinase substrate peptide (compound 45) is a potent and selective inhibitor of ATR (Ataxia Telangiectasia and Rad3 related) protein kinase ( K i =6 nM). ATR kinase substrate peptide inhibits ATR activity by competing with ATR kinase ATP-binding sites to block ATR mediated signaling. ATR kinase substrate peptide can be used to study the role of ATR kinase in DNA damage response.