DNA double-strand breaks originating from diverse causes in eukaryotic cells are accompanied by the formation of phosphorylated H2AX (␥H2AX) foci. Here we show that ␥H2AX formation is also a cellular response to topoisomerase I cleavage complexes known to induce DNA double-strand breaks during replication. In HCT116 human carcinoma cells exposed to the topoisomerase I inhibitor camptothecin, the resulting ␥H2AX formation can be prevented with the phosphatidylinositol 3-OH kinase-related kinase inhibitor wortmannin; however, in contrast to ionizing radiation, only camptothecin-induced ␥H2AX formation can be prevented with the DNA replication inhibitor aphidicolin and enhanced with the checkpoint abrogator 7-hydroxystaurosporine. This ␥H2AX formation is suppressed in ATR (ataxia telangiectasia and Rad3-related) deficient cells and markedly decreased in DNA-dependent protein kinase-deficient cells but is not abrogated in ataxia telangiectasia cells, indicating that ATR and DNA-dependent protein kinase are the kinases primarily involved in ␥H2AX formation at the sites of replication-mediated DNA doublestrand breaks. Mre11-and Nbs1-deficient cells are still able to form ␥H2AX. However, H2AX؊/؊ mouse embryonic fibroblasts exposed to camptothecin fail to form Mre11, Rad50, and Nbs1 foci and are hypersensitive to camptothecin. These results demonstrate a conserved ␥H2AX response for double-strand breaks induced by replication fork collision. ␥H2AX foci are required for recruiting repair and checkpoint protein complexes to the replication break sites.Compact chromatin can be a structural barrier for DNA processing during replication, transcription, recombination, and DNA repair. Following DNA damage, chromatin must be modified to permit the access of repair proteins to the DNA lesions (1). Homologous recombination and non-homologous end joining are the main repair pathways for DNA doublestrand breaks (2). Both processes are assumed to require chromatin alterations for DNA end processing, strand invasion, branch migration, DNA synthesis, and ligation as well as for recruiting checkpoint proteins (3).The fundamental unit of chromatin is the nucleosome, which consists of an octamer of the four core histones, H2A, H2B, H3, and H4, around which the DNA is bound. Histone H2A includes three subfamilies whose members contain characteristic sequence elements that have been conserved independently throughout eukaryotic evolution: H2A1-H2A2, H2AZ, and H2AX (4 -6). In mammals, H2AX represents 2-25% total H2A, whereas in yeast, the major H2A is the H2AX ortholog (7,8).H2AX phosphorylation (at its C terminus on serine 139) has been found at the sites of double-strand breaks in chromosomal DNA (9). This phosphorylated form of H2AX has been named ␥H2AX 1 (8). ␥H2AX is rapidly formed in cells treated with ionizing radiation (IR) but also during V(D)J and class-switch recombination and apoptosis (8 -13). Because ␥H2AX appears within minutes after IR, ␥H2AX focus formation is considered to be a sensitive and selective signal for the existe...

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Cross-reactivity between antibodies to different human coronaviruses (HCoVs) has not been systematically studied. By use of Western blot analysis, indirect immunofluorescence assay (IFA), and enzyme-linked immunosorbent assay (ELISA), antigenic cross-reactivity between severe acute respiratory syndrome (SARS)-associated coronavirus (SARS-CoV) and 2 HCoVs (229E and OC43) was demonstrated in immunized animals and human serum. In 5 of 11 and 10 of 11 patients with SARS, paired serum samples showed a > or =4-fold increase in antibody titers against HCoV-229E and HCoV-OC43, respectively, by IFA. Overall, serum samples from convalescent patients who had SARS had a 1-way cross-reactivity with the 2 known HCoVs. Antigens of SARS-CoV and HCoV-OC43 were more cross-reactive than were those of SARS-CoV and HCoV-229E.