A, Early, late and late-passage HFF transduced with a hTERT-expressing retrovirus vector were irradiated with 0.4 Gy and analyzed by immunofluorescence with anti-γH2AX at the indicated time points post-IR. Cells were considered positive if 3 or more foci were present. B, Kinetics of H2AX phosphorylation after 1 Gy IR was assessed by immunoblot with anti-γH2AX in early, late, and late-passage cells transduced with the hTERT retrovirus.