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