While studying how the heart is formed, researchers at the University of Pennsylvania School of Medicine serendipitously found a novel cellular source of atrial fibrillation (AF), the most common type of abnormal heart beat. Jonathan Epstein, MD, William Wikoff Smith Professor, and Chair, Department of Cell and Developmental Biology, and Vickas Patel, MD, PhD, Assistant Professor of Medicine, have identified a population of cells in the atria of the heart and pulmonary veins of humans and mice that appear to be the seat of AF. The finding may lead to a more precise way to treat AF, with reduced side effects. Their findings appear online in the Journal of Clinical Investigation.