Numerous studies have shown that induction of cardiomyocyte cell cycle activity can have a profound beneficial impact on cardiac structure and function following myocardial infarction. It has also been shown that genetic background can impact the intrinsic rate of cardiomyocyte cell cycle activity in mice. We have observed that mice in a DBA/2J genetic background (abbreviated DBA) have very low levels of cardiomyocyte cell cycle activity following myocardial infarction. However, when crossed with C57Bl6/NCR mice (abbreviated NCR), the resulting (DBA x NCR)-F1 animals mice exhibit a marked increase in cardiomyocyte S-phase activity following infarction, indicating the presence of an autosomal gene (or genes) in the NCR background which acts in a dominant manner to facilitate cell cycle re-entry. Analysis of backcross mice established that this gene (or genes) resides in a region of interest (ROI) located on the distal end of chromosome 3. The experiments proposed in Aim 1 will test hypothesis that a single gene within the ROI is responsible for elevated cardiomyocyte S-phase activity post-infarction. Candidate genes within this region will be identified based on expression patterns observed in infarcted DBA vs. NCR hearts as well as by the presence of sequence variants predicted to impact protein structure and/or activity. The candidates will be systematically tested by generating genetically modified animals, subjecting them to myocardial infarction, and then monitoring the level of cardiomyocyte S-phase activity; an induction of cardiomyocyte cell cycle activity would confirm that the candidate gene being tested is responsible for the trait. The experiments proposed in Aim 2 will test the hypothesis that the elevated cell cycle activity encoded by the NCR ROI alleles has a positive impact on the diminished cardiac function and adverse myocardial remodeling which is encountered post-infarction. Congenic mice in a DBA genetic background which retain heterozygosity on the distal end of chromosome 3 and thus carry the NCR allele (or alleles) which is a major contributor to cardiomyocyte S-phase induction will be generated. The mice will then be subjected to myocardial infarction and longitudinal functional analysis. Terminal analyses will include comprehensive hemodynamic measurements as well as assessment of adverse remodeling (cardiomyocyte apoptosis, hypertrophy and myocardial fibrosis); relative improvements in cardiac function and structure would indicate a beneficial effect from the NCR-encoded cell cycle activity following myocardial injury. Ultimately, the identification and validation of genes underlying intrinsic differences in cardiomyocyte cell cycle rates observed in different strains of mice could suggest potential therapeutic targets with which to enhance regenerative growth in injured hearts.