PROJECT SUMMARY/ABSTRACT Dogma in the cardiovascular field argues that the adult mammalian heart is essentially non-regenerative and that this failure to regenerate is primarily attributed to the post-mitotic and polyploid nature of most cardiomyocytes (CMs). Multiple pieces of evidence now support the idea that within the adult mammalian myocardium, mononuclear diploid cardiomyocytes (MNDCMs) are a privileged subpopulation of CMs that have avoided this proliferative senescence. This attribute confers a unique capacity to re-enter the cell cycle and regenerate myocardial tissue. Our recent work in mice demonstrates that the frequency of MNDCMs and the competence to regenerate one's heart are two interlinked and variable traits influenced by the complex genetic background of an individual. In other words, contrary to longstanding beliefs, some individuals can mount a meaningful regenerative response after an insult, such as a myocardial infarction. We then took a genome- wide association strategy to identify the genes associated with the observed variation. From this analysis, we identified Tnni3k as one candidate that regulates CM senescence by inhibiting cytokinesis, specifically. The parent R01, examines two additional candidate genes that arose from the original screen, each of which has a unique effect on CM cell cycle and ploidy. Experiments proposed in the parent grant investigate both candidates for their effects on CM ploidy, cell cycle, and post-injury outcomes, specifically in the context of an adult injury setting. This Diversity Supplement proposes additional experiments which will strengthen the second aim of the parent award and provide complementary evidence in support of candidate gene, Runx1. Specifically, using the same gain- and loss-of-function genetic mice from the parent award, we extend our analysis of Runx1 into an injury model that displays substantial CM proliferation and competence for myocardial regeneration, the neonatal regeneration model. Thus, allowing us to examine the effect of Runx1 in both a pro- and non-regenerative model of heart injury.