The present invention relates to human Rif1 (hRif1) nucleic acid sequences and human Rif1 amino acid sequences encoded therefrom. The invention also encompasses antibodies that are immunologically specific for human Rif1 polypeptides. Also included in the present invention are methods directed to detecting Rif1 foci formed in response to various treatments that induce DNA damage. The formation of foci comprising Rif1 in response to DNA damage serves as a positive indicator of the presence of wild type ATM kinase activity in a cell, whereas the absence of detectable Rif1 foci under such circumstances serves to identify a cell with reduced ATM kinase activity. Modulators of ATM kinase activity and/or Rif1 activity and methods for identifying such modulators are also included in the present invention.