library_construction_protocol: Hearts were isolated at 3 dpa together with the uninjured control (2 biological replicates each) on ice and washed several times to remove blood cells.  Ventricles were digested in an Eppendorf tube with 0.5 ml HBSS plus 0.13 U/ml Liberase DH (Roche) at 37°C, while stirring gently with a Spinbar® magnetic stirring bar (Bel-Art Products).  Supernatants were collected every 5 min and neutralized with sheep serum.  Dissociated cells were spun down and re-suspended in DMEM plus 10% fetal bovine serum (FBS) medium and sorted using a Sony MA900 sorter for EGFP+ cells (uninjured) and EGFP+mScarlet+ (3 dpa). Nuclei preparation and CUT&TAG library preparation using rabbit anti-H3K4me1 antibody (Abcam, ab8895) were performed using the EpiCypher CUTANA™ CUT&Tag Kit following the manual. Two replicates with 5,000-12,000 nuclei for each sample were used.