ChIPs for H3K9me3S10ph and Atrx were performed with approximately 2x107 neurons per sample in biological triplicates/group. Primary cortical neurons +/- isotonic KCl stimulation (5 hr) were crosslinked with 1% paraformaldehyde (PFA) for 12 min at room temperature and then quenched for five min with 0.125 M glycine. Pelleted cells were washed 5X with 1X PBS containing protease inhibitors before being subjected to lysis, sonication, IP (7.5 μg antibody per sample was used) and DNA purification, as previously described. Following DNA purification, ChIP-seq libraries were prepared according to the Illumina protocol and sequenced with the HiSeq 2000.