Once the heart is formed, its muscle cells have very limited ability to regenerate. After a heart attack, these cells die off and scar tissue forms, potentially setting people up for heart failure. A new study advances the possibility of using microRNAs — small molecules that regulate gene function — to regenerate heart muscle. In mice, two microRNAs that are abundant in developing hearts, miR-19a and miR-19b, repaired heart muscle and improved cardiac function after heart attack.