Circulation. 2017; 136: 680–686. DOI: 10.1161/CIRCULATIONAHA. 117.029343 August 15, 2017 681 proliferation. 5 It remains unclear whether this regenerative window exists in large animals or in humans. iii. Although cardiomyocytes appear to continue to renew throughout life, the quantitatively dominant mechanism of growth in the mammalian postnatal heart is an increase in cardiomyocyte size (reviewed by Heineke and Molkentin6). iv. In the healthy, uninjured adult human and murine heart, the total number of cardiomyocytes remains essentially stable, and cardiomyocyte turnover is currently estimated at 0.5% to 2% per year in both species. 4, 7–9 B. After cardiac injury in adult mammals i. Cardiomyocyte renewal rates may be higher after injury than under normal conditions. 9 ii. The experimental determination of cardiomyocyte turnover after cardiac injury can be challenging owing to inflammation, proliferation of stromal and vascular cells, and scar formation.