A method for detecting DNA damage in a tissue sample involves contacting an immobilized biological sample with a labeled ligand which binds to human 53Bp1, and examining the immobilized sample for the presence of a label generated-detectable signal concentrated in foci in said sample. The presence of concentrated foci is indicative of DNA damage and the presence of diffuse signal is indicative of a normal sample. Diagnostic reagents contain a ligand that binds to human 53Bp1 associated with a detectable label. Diagnostic kits for detecting DNA damage in a biological sample contain such diagnostic reagents and signal detection components. Compositions that inhibit or antagonize the biological activity of 53Bp1 are identified by suitable assays, and are employed in methods of retarding the growth of a cancer cell.