Much is known about the distal DNA damage repair response. In particular, many of the enzymes and auxiliary proteins that participate in DNA repair have been characterized. In addition, knowledge of signaling pathways activated in response to DNA damage is increasing. In contrast, comparatively less is known of DNA damage-sensing molecules or of the specific alterations to chromatin structure recognized by such DNA damage sensors. Thus, precisely how chromatin structure is altered in response to DNA damage and how such alterations regulate DNA repair processes remain important unanswered questions. In vertebrates, phosphorylation of the histone variant H2A.X occurs rapidly after double-strand break formation, extends over megabase chromatin domains, and is required for stable accumulation of repair proteins at damage foci. We have shown that reactive oxygen species (ROS)-induced DNA single-strand breaks induce the incorporation of P specifically into histone H3. ADP-Ribosylation of histones may stimulate local chromatin relaxation to facilitate the repair process, and, indeed, histone ribosylation preceded DNA damage-induced histone H3 phosphorylation. However, H3 phosphorylation occurred concomitant with overall chromatin condensation, as revealed by decreased sensitivity of chromatin to digestion by micrococcal nuclease and by DAPI staining of nuclei. Inhibitors of the ERK and p38MAPK pathways and inhibition of poly(ADP-ribose) polymerase all reduced ROS-induced H3 phosphorylation, chromatin condensation, and cell death. Precisely how changes in the post-translational modification of histone H3 regulate the survival response remains unclear. Attempts to determine the precise site of histone H3 phosphorylation, putative histone H3 kinases, and histone H3 interacting proteins are underway. Copyright © Informa Healthcare.