library_construction_protocol: Hearts were dissected, pooled by genotype, and dissociated in 200ug/mL Liberase TM (Sigma-Aldrich) dissolved in Hibernate E minus calcium media (Transnetyx) for 30 minutes at 37°C Single-cell suspensions loaded onto a 10X Genomics Next GEM Chip and emulsified with 3' Single Cell Next GEM beads using a ChromiumTM Controller (10X Genomics). All embryos were dissected, digested, and barcoded on the same day to minimize variation. From barcoded cDNAs, gene expression libraries were constructed using ChromiumTM Single Cell 3ʹ Library Kits (10X Genomics) at the Translational Science Laboratory (Medical University of South Carolina). Next-generation sequencing was performed on each sample using an Illumina NovaSeq S4 flow cell at the VANTAGE facility (Vanderbilt University Medical Center)