Impact of Mrad9 status on formation of chromatid and chromosome aberrations. Chromatid (A) and chromosome (B) aberration frequencies in unirradiated control (open bars), irradiated (closed bars) and bystander (stippled bars) mouse embryonic stem cells as a function of Mrad9 status (Mrad9+/+, Mrad9−/− or the latter ectopically expressing Mrad9+) in mouse embryonic stem cells at 7 days post irradiation (mean ± SD; n = 2). Asterisk and double asterisk depict values that are statistically significant, p < 0.1 and p < 0.05, respectively, between the controls and the corresponding experimental groups. Results from two experiments were pooled and are expressed as mean ± SD. Differences in these data were analyzed using Student’s t-test. (Additional file2 shows representative pictures of metaphase spreads and aberrations scored in this study).