NOX4 (NADPH oxidase 4) is an enzyme from the NADPH oxidase family that produces reactive oxygen species (ROS) inside cells. Unlike other members of this family, NOX4 remains continuously active under normal conditions. It is primarily located in the endoplasmic reticulum and plasma membrane, where it transfers electrons from NADPH to oxygen, producing superoxide radicals. The ROS generated by NOX4 function as signaling molecules involved in cell growth, differentiation, and programmed cell death, among other physiological processes.