Induction of ATM activation and H2AX phosphorylation in A549 cells following their exposure to IM16 CS. Bivariate distributions (scattergrams; DNA content, DI vs. ATM-S1981P IF, or vs. γH2AX IF, respectively) of the A549 cells, mock treated (Ctrl) or exposed 20 min to CS (CS), as described in the Materials and Methods. After exposure to CS or mock treatment the cells were incubated for an additional hour, then fixed and subjected to immunocytochemical detection of ATM-S1981P or γH2AX, their DNA counterstained with DAPI and the intensity of IF- and DAPI-fluorescence measured by iCys. The inset shows the cellular DNA content histogram representing cells from these cultures. Based on differences in DI the cells can be subdivided into G1, S and G2M compartments, as shown in the left panel. Mitotic (M) A549 cells constitutively express higher level of ATM-S1981P and γH2AX [14,38,70,71] and their position on the scattergrams is marked within dashed oval boundaries. The dashed line represents the upper threshold of ATM-S1981Por γH2AX IF level for 97% of interphase cells.