An imbalance between atrial nitric oxide (NO) and ROS production has been associated with the new onset of atrial fibrillation (AF); in particular, the activity of atrial ROS-generating enzymes, such as NOX2-containing NADPH oxidases, is independently associated with postoperative AF in patients undergoing cardiac surgery,1,8–10 whereas knockout of the neuronal NOS isoform increases the probability of AF induction in mice.11 The gene discussed is FMO5; the disease is atrial fibrillation.