Heart muscle cannot renew itself very well. Researchers would like to help that process by finding populations of cells in the heart that can do so, and then boosting that capacity. The two Nature papers aim to get to the heart of the matter. In one, a team led by Richard Lee at the Brigham and Women’s Hospital and Harvard Medical School, both in Boston, Massachusetts, traced the birth and fate of heart muscle cells in mice. Lee and his colleagues found that a small proportion of heart cells — less than 1% — can regenerate themselves normally. After a heart attack that proportion goes up, but only to 3%.