A study published in *Nature* (2005) identifies isl1⁺ cardiac progenitors in postnatal rat, mouse, and human myocardium. These cells, termed cardioblasts, can differentiate into mature cardiomyocytes without cell fusion, maintaining functional characteristics such as calcium cycling and action potentials. The research demonstrates that isl1⁺ cells are endogenous cardiac progenitors that contribute to the embryonic heart and persist in the postnatal heart, with their numbers decreasing over time. These cells are localized in specific cardiac regions and can be purified and expanded in culture. Using genetic marking techniques, the study shows that isl1⁺ cells can be selectively labeled and traced during development, revealing their role in cardiac lineage formation. The study also shows that isl1⁺ cells can differentiate into mature cardiomyocytes in co-culture with neonatal myocytes, indicating their potential for cardiac regeneration. The findings suggest that isl1⁺ cells represent a genetically defined system for studying cardiac cell lineage formation and maturation, with implications for understanding congenital and adult cardiac diseases. The study highlights the importance of isl1 in cardiac development and provides insights into the mechanisms of cardiac progenitor cell differentiation and function.A study published in *Nature* (2005) identifies isl1⁺ cardiac progenitors in postnatal rat, mouse, and human myocardium. These cells, termed cardioblasts, can differentiate into mature cardiomyocytes without cell fusion, maintaining functional characteristics such as calcium cycling and action potentials. The research demonstrates that isl1⁺ cells are endogenous cardiac progenitors that contribute to the embryonic heart and persist in the postnatal heart, with their numbers decreasing over time. These cells are localized in specific cardiac regions and can be purified and expanded in culture. Using genetic marking techniques, the study shows that isl1⁺ cells can be selectively labeled and traced during development, revealing their role in cardiac lineage formation. The study also shows that isl1⁺ cells can differentiate into mature cardiomyocytes in co-culture with neonatal myocytes, indicating their potential for cardiac regeneration. The findings suggest that isl1⁺ cells represent a genetically defined system for studying cardiac cell lineage formation and maturation, with implications for understanding congenital and adult cardiac diseases. The study highlights the importance of isl1 in cardiac development and provides insights into the mechanisms of cardiac progenitor cell differentiation and function.