By investigating murine heart 14 days after MI with genetic lineage tracing using Ki67 knockin models, no cycling CMs were found in infarcted hearts. A subpopulation of proliferative fibroblasts expressing Fstl1 was identified, which resembled neonatal cardiac fibroblasts. Conditional KO of Fstl1 in fibroblasts resulted in more rupture. The gene discussed is FSTL1; the disease is myocardial infarction.