Expression levels of Nox4 are increased in a number of diseases including fibrosis, PAH and stroke, but the functional role ascribed to Nox4 is frequently both deleterious and beneficial (Brandes et al., 2011; Altenhofer et al., 2012; Schmidt et al., 2012; Schroder et al., 2012). Here, NOX4 is linked to pulmonary arterial hypertension.