In conclusion, we show for the first time in hPASMCs from PAH patients that serotonin increases Nox1-dependent ROS generation and decreases Nrf-2-antioxidant systems through c-Src–dependent processes that contribute to oxidation of proteins, DNA damage, and redox-sensitive proliferation and fibrosis of hPASMCs, processes involved in PAH. The gene discussed is SRC; the disease is pulmonary arterial hypertension.