Hearts from euthanized three-week-old male mice were rapidly harvested, rinsed in cold phosphate buffered saline, and snap frozen in liquid nitrogen. Genomic DNA was extracted from frozen tissue using the Zymo-Spin ChIP Kit according to manufacturer's directions. Chromatin was immunoprecipitated in triplicate using antibodies against RNA Pol-II (Bethyl Laboratories #9201012), H3K27ac (Abcam #ab4729), H3K27me3 (Abcam #ab6002), and normal mouse IgG (Diagenode #C15410206 and Millipore #12-371B). Libraries were prepared from pooled ChIP DNA and constructed by Zymo Research using EpiQuest services and proprietary methods.