Frequency of telomere-DSB fusions in the 6 mouse strains. The C.B6 and LEWES strains showed low levels of telomere/DSB fusions after exposure to 1 Gy γ rays, similar to C57BL/6, and consistent with wild-type DNA-PKcs. The SCID, BALB/c and B6.C strains had significantly higher levels of telomere/DSB fusions after irradiation (*P < 0.0005, #P < 0.0001, 0 Gy compared to 1 Gy), consistent with mutant DNA-PKcs.