PECAM1 and atrial fibrillation: We found a few isolated cells positive for the endothelial marker CD31 (SR: 24/585, 3.8 ± 0.8%, n = 3; AF: 18/714, 1.8 ± 1.6%, n = 4) which implies that the rare endothelial cells that migrated out of the tissue did not proliferate, and further confirms that the cell population consisted mostly of fibroblasts.