The ATMR-NLS reporter is specific for ATM activity. (a) 293-Luc, ATMR-NLS, or ATMR-NLSmut cells were treated with increasing concentrations KU-55933 for 1hr. ATMR-NLS cells show a dose-dependent increase in reporter activity compared to other reporters (top panel shows representative image). (b) Treatment of 293-ATMR-NLS cells with ATM siRNA caused target knockdown, as demonstrated by Western blots. 72 hours after siRNA transfection, cells were treated with 2 Gy radiation, and subjected to bioluminescence assays at the indicated time points. Pre-treatment with ATM siRNA resulted in substantial decline in reporter response after treatment with radiation (see black arrows). For both experiments (a) and (b), data are normalized to values from 293-Luc (control) cells, and asterisks indicate p-value < 0.05 compared to control cells. Error bars s.e.m.