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Cover Structural maintenance of chromosomes protein 1 (SMC1) is phosphorylated in cells following exposure to ionizing radiation (IR). Shown here are metaphase chromosome spreads of wild-type (pink) and Smc1-phosphorylation mutant (blue) mouse embryonic fibroblast (MEF) cells, which were treated with colcemid 1 h after 1Gy of IR. Phosphorylated SMC1 is localized to sites of double-strand DNA breaks, and is required for proper activation of DNA repair pathways. Also shown are immunofluorescence analyses of wild-type MEF cells 30 min after irradiation with 5Gy of IR, using antibodies directed against phosphorylated SMC1 (green). Nuclei were stained with DAPI (red; color adjusted by author). (For details,see Kitagawa et al., 1423.)