Slides were deparaffinized in xylene and rehydrated in graded ethanol. Antigen retrieval was performed with Envision™ FLEX Target Retrieval Solution, pH 9.0, in a 98°C water bath for 10 minutes for ATM and 45 minutes for MRE11. This was followed by incubation for 5 minutes at room temperature with Envision™ FLEX Peroxidase-Blocking Reagent to block endogenous peroxidases. Slides were incubated with monoclonal anti-ATM primary antibody [2C1 (1A1)] (1:800 dilution, Abcam, Cambridge, UK) for 30 minutes at room temperature and with anti-MRE11 primary monoclonal mouse antibody (1:600 dilution, ab214, Abcam). After washing with TBST buffer, slides were incubated for 15 minutes with Dako mouse linker, rinsed, and incubated for 30 minutes with anti-mouse secondary antibody. The peroxidase substrate used was a mixture of Envision™ FLEX DAB+Chromogen DM827 and Envision™ FLEX Substrate Buffer DM823 (DAKO). Slides were counterstained with hematoxylin, washed with cold water, and then dipped 10 times in Scott Bluing solution. Slides were immediately rinsed with cold water before dehydration and mounting.