Isolated lymphocytes were spotted onto microscope slides (six slides/each individual) at an optimized cell density for immunofluorescence staining. Cells were fixed in 3.7% paraformaldehyde in PBS (15 min at room temperature) and permeabilised using 0.5% Triton X 100 in PBS (5 min at 4°C). Samples were blocked in phosphate buffered saline (PBS) with 2% bovine serum albumin (BSA) for 3 × 5 min at room temperature. Samples were then incubated with anti-γH2AX antibody (clone JBW301) overnight at 4°C, and washed in PBS, 2% BSA, for 2 × 5 min. Finally slides were incubated with FITC-conjugated goat anti-mouse secondary antibody (Invitrogen, Paisley, UK) for 1 h at room temperature. Slides were washed in PBS for 2 × 2 min, stained with 4,6-diamidino-2-phenylindole (DAPI) for 5 min, washed for 2 min in PBS, and mounted using Mowiol Mounting Medium.