Induction of H2AX phosphorylation following exposure to CS. Photomicrographs of the mock treated cells (A, B) or cells exposed for 20 min to IM16 CS (C, D). The cells were immunocytochemically labeled with phospho-specific γH2AX antibody, with the secondary antibody fluorescing green (A, C); their DNA was counterstained with DAPI (B, D). Note the low level of constitutive H2AX phosphorylation in the mock-treated cells with discrete punctuate pattern reflecting the presence of IF foci, consisted with the data presented before [70,77]. The CS treated cells show intense γH2AX IF due to very large numbers of IF foci located close to each other. Photomicrographs of the CS treated cells were obtained using automatic camera exposure. To demonstrate the much lower γH2AX IF of mock treated cells, the camera aperture was opened and closed manually for an equivalent length of time.