es cell lines including the g9a knockout were cultured based on the standard protocol of the es core laboratory at johns hopkins university ( see full methods for details ) . we differentiated es cells for 1824 days by removing leukemia inhibitory factor ( lif , chemicon ) from the medium and culturing in plates without feeder cells . chromatin was isolated without sonication or formalin cross linking , by digested with micrococcal nuclease ( mnase , ge healthcare , piscataway , nj ) , and then chromatin immunoprecipitation was performed with an antibody specific to h3k9me2 ( abcam , cambridge , ma ) . chip and input dna were amplified , labeled by cy5 and cy3 respectively , and hybridized to nimblegen tilling arrays as described4 . we first conducted within - array normalization to correct the probe sequence effects and position bias . locks are defined as the genomic regions where smoothed values are above 97.5th quantile of log ratios in reference regions ( see full methods for details ) . the pdf files containing all the pictures of locks can be obtained through our website : http://rafalab.jhsph.edu / k9locks/. es cell lines including the g9a knockout were cultured based on the standard protocol of the es core laboratory at johns hopkins university ( see full methods for details ) . we differentiated es cells for 1824 days by removing leukemia inhibitory factor ( lif , chemicon ) from the medium and culturing in plates without feeder cells . chromatin was isolated without sonication or formalin cross linking , by digested with micrococcal nuclease ( mnase , ge healthcare , piscataway , nj ) , and then chromatin immunoprecipitation was performed with an antibody specific to h3k9me2 ( abcam , cambridge , ma ) . chip and input dna were amplified , labeled by cy5 and cy3 respectively , and hybridized to nimblegen tilling arrays as described4 . we first conducted within - array normalization to correct the probe sequence effects and position bias . locks are defined as the genomic regions where smoothed values are above 97.5th quantile of log ratios in reference regions ( see full methods for details ) . the pdf files containing all the pictures of locks can be obtained through our website : http://rafalab.jhsph.edu / k9locks/.