secondary antibody (A-11001, Life Technologies) diluted 1:500 in PBS, 1% BSA, 0.5% Tween-20 for 6 h at room temperature. The slides were then washed three times in PBS, 1% BSA, 0.5% Tween-20 for 15 min and stained with Hoechst 33342 trihydrochloride (H3570, Molecular Probes) diluted 1:2000 in PBS for 15 min at room temperature. The slides were then washed three times in PBS for 10 min on each occasion, washed once in water for 5 min, mounted with ProLong Gold Antifade Mountant (P36930, Molecular Probes) and coverslips were lowered onto the slides. Thirty metaphases were captured per sample using a Zeiss LSM 780 confocal microscope (Zeiss). The number of sister-chromatid exchanges per metaphase was then counted blind.
