In PD, reactive oxygen species (ROS) arise from multiple intracellular sources, among which mitochondrial dysfunction is widely recognized as a fundamental driver, while nicotinamide adenine dinucleotide phosphate (NADPH) oxidase 4 (NOX4), a constitutively active NOX isoform that predominantly generates hydrogen peroxide (H<sub>2</sub>O<sub>2</sub>), has emerged as an important enzymatic contributor in the central nervous system. Here, NOX4 is linked to Parkinson disease.