Fig. S7 displays an additional ChIP experiment (corresponding to Fig. 6) showing that H3K9me3 and KAP1 mark the donor switch region (S) be- fore and during CSR. Table S1 shows the two-color IgH-FISH analysis. Tables S2, S3, and S4, included as separate PDF files, show the statistical analyses corresponding to the ChIP experiments in Fig. 6, Fig. S6, and Fig. S7, respectively. Table S5 shows the primers used in this study. Table S6 shows the antibodies used in this study. Online supplemental material is available at http://www.jem.org/cgi/content/full/jem.20110118/DC1. We thank members of the Losson and Reina-San-Martin laboratories for discussions and Evi Soutoglou and Olivier Pourquié for comments on the manuscript. We also thank Klaus Rajewsky for CD19Cre mice, Michel Nussenzweig for the anti-AID antibody for ChIP analysis, Anna Gazumyan for advice on AID-ChIP, Raphaël Riclet for advice on H3K9me3, KAP1, and HP1 ChIP, Manuela Argentini and Frank Ruffenach for assistance in mass spectrometry analysis, Isabelle Billas for advice on gel filtration chromatography, Sonia Beroud for assistance with switch junction analysis, Claudine Ebel for assistance with cell sorting, Pascal Eberling for peptide synthesis, and Michael Gendron for animal care and immunizations. B.P. Jeevan-Raj was supported by fellowships from the Association pour la Recherche sur le Cancer (ARC) and the Fondation pour la Recherche Médicale. F.W. Alt is an investigator of the Howard Hughes Medical Institute. This work was supported by grants to B. Reina-San-Martin from the Agence Nationale pour la Recherche (ANR-07-MIME-004-01), the ARC, and the Institut National de la Santé et de la Recherche Médicale (INSERM). B. 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