MCF7 cells were transferred to hypoxic culturing conditions for the indicated durations and immediately fixed to avoid reoxygenation. After 24 hours of hypoxia, cells were reoxygenated for 8 hours. Cells were disrupted by sonication, yielding genomic DNA fragments ranging from 200-1000 bp, with a bulk size of 200-500bp. For each immunoprecipitation 10-20 million cells were used. ChIPs were performed and analyzed using standard protocols (antibody: H3K4me3 and H3K27me3 (07-449; Upstate)). ChIP-seq libraries were prepared for sequencing using standard 36 bp paired-end protocol for the Illumina Genome Analyser IIx (GAIIx). Two lanes were used per sample for sufficient coverage.