The ventricles (30 adult ventricles per sample) were gently chopped, transferred, and digested in digestion buffer for 15 min at 4℃ with constant gentle agitation using a rotator. The digestion was then neutralized with 10% FBS (fetal bovine serum) and the disassociated cells were filtered through a 100 μm-strainer to remove large clumps and pelleted at 500 × g for 10 min at 4℃. The pellet which mainly contained platelets and erythrocytes was re-suspended in Hank's balance salt solution (HBSS). Then, around 2000,000 cells per sample were fixed in 0.2% formaldehyde for 2 min and neutralized by adding glycine to a final concentration 25 mM. The fixed cells were pelleted at 800 × g for 5 min at 4℃, washed twice with PBS and loaded for CUT&TAG reaction. In brief, the cells were incubated with primary antibody overnight. Next day, the cells were washed twice and incubated with second antibody with Tn5 transposase. The cells were lysed and antibody binding DNA fragments were extracted. The extracted DNA fragments were ligated with double index primers and amplified to generate sequencing library. The library was extracted and qualified using Qubit and Fragment Analyzer.