library_construction_protocol: Hearts were excised and digested via retrograde perfusion with Liberase as described in Rudolph et. al. 2019 (https://doi.org/10.1073/pnas.1904385116). In labeled samples 10mM 4sU was addedised, minced and dissociated py pipetting. Cardiomyocyted were sedimented by gravity any the pellett resuspended in TRIzol. In labeled samples 10mM 4sU was added to the buffers from mincing to sedimentation. 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!