library_construction_protocol: Embryonic hearts were microdissected away from the embryo, minced with a scalpel in PBS w/ 10mM 4sU and transferred into pre‑warmed 0.25% trypsin‑EDTA (Thermo Fisher Scientific #25200056) and 10 mM 4sU. The hearts were digested for 15 min at 37°C with several rounds of vigorous pipetting and then spun down. The cell pellet was washed twice in 20% FCS in PBS solution, centrifuged and resuspended in 1 ml PBS. The remaining cell suspension was incubated for 30 min in the dark with 1 µl reconstituted fluorescent reactive dye (Thermo Fisher Scientific #L34955) to determine cell viability. After another wash in PBS, cells were resuspended in 0.5 ml 5% FCS in PBS solution. The cell suspensions were then incubated with 1 µg/ml Alexa Fluor 647 Rat Anti-Mouse VCAM-1 (BD Biosciences #561612) for 15 min in the dark. The cells were centrifuged at 450 g for 5 min, resuspended in PBS and filtered using a 70 µm strainer into a polypropylene tube. Live, VCAM-1+ cardiomyocytes were sorted using a BD FACSAria II (BD Biosciences), transferred to 1.5 ml tubes and centrifuged at 450 g for 5 min. The cell pellets were resuspended in 1 ml TRIzol RNA was extracted using TRIzol-Chloroform-Isopropanol according to the manufacturer's instructions. Iodiacetamide Treatment was carried out on extracted RNA in 50% DMSO as described in Herzog et. Al 2017 (https://doi.org/10.1038/nmeth.4435). libraries for RNA-seq were prepared following a CELseq protocol as published in Holler & Junker 2019 (https://doi.org/10.1007/978-1-4939-9009-2_9). Sequencing adapters were switched – Insert is in read 1!