Cardiac fibroblasts, Tcf21 lineage GFP positive, were isolated from hearts by langendorff perfusion. Sorted cells were spun down and lysed prior to tagmentation according to the Fast-ATAC-seq protocol (Corces et al., 2016). Briefly For Fast-ATAC (ATAC-seq), sorted cells were spun down, FACS buffer was removed, the pellet was then re-suspended in a transposase-containing reaction mixture complete with 0.05% digitonin prior to tagmentation at 370C with 1000 rpm agitation for 30 minutes. Next, transposed DNA was purified with a Qiagen PCR MinElute kit (Qiagen 28004). Fast-ATAC libraries were purified with a 1.8X SPR purification using AMPure XP beads following PCR amplification.