DNA damage precedes cell death. Chronological relationship among γH2AX, caspase-3 activation, and viability of AxCAhtt111Q-infected primary cortical neurons. Caspase-3 activation was detected with anti–activated caspase-3 antibody that detects only the cleaved form of caspase-3. Activation of γH2AX started at day 2 and preceded that of caspase-3 and cell death after day 5. Viability of neurons was evaluated by trypan blue staining. (A) Representative Western blots. (B) Quantitative analysis of signal intensities of γH2AX and active caspase-3 and their chronological relationship. (C) Viability of primary cortical neurons expressing mutant Htt was estimated by trypan blue. Together with A–C, H2AX phosphorylation clearly precedes caspase-3 activation and cell death. Error bars indicate SEM.