Isolated hearts were dissociated into a single cell suspension. Isolated hearts were centrifuged at 1,300 rpm for 5 minutes at 4°. Supernatant was removed and hearts were washed 1x with 1x PBS followed by centrifugation at 1,300 rpm for 5 min at 4°. Hearts were then suspended in a solution of 440 µL trypsin (Thermo Fisher; 15400054) and 60 µL Liberase (Millipore Sigma; 5401020001) and placed in a 30° heat block and triturated with a pipette every 2 minutes until hearts were dissociated (~15 minutes). After the hearts were dissociated, 500 µL of EDM was added to stop the dissociation, followed by centrifugation at 2,300 rpm for 5 min at 4°. Supernatant was removed and cells were resuspended in 1mL 1x PBS + 1% FBS. Cells were centrifuged at 2,300 rpm at 4° for 5 minutes, resuspended in 1 mL 1x PBS + 1% FBS, and applied to a 40 µm filter (pluriSelect; 43-10040-60) in a 1.5 mL tube. Single cells were concentrated by centrifugation at 2,300 rpm at 4° for 5 minutes and then resuspended in 50 µL. This single cell preparation was submitted to the CCHMC Gene Expression Core. The libraries were prepared for sequencing using the 10x Genomics Chromium platform.