Nox4 is the major NADPH oxidase homolog expressed in human pulmonary artery smooth muscle cells (Sturrock et al., 2006), and its expression both at the mRNA and protein level is significantly increased in lungs from patients with idiopathic pulmonary arterial hypertension (IPAH) compared to healthy lungs (Mittal et al., 2007), which suggests a correlation between Nox4 and the onset of PAH. The gene discussed is FMO5; the disease is idiopathic pulmonary arterial hypertension.