The integral membrane protein flavocytochrome b (Cyt b), a heterodimer formed by a 91-kD glycoprotein (heavy chain) and a 22-kD polypeptide (light chain), is the catalytic core of the phagocyte NADPH oxidase and generates superoxide which plays a critical role in host defense. The p22phox gene encodes the light alpha subunit which has been proposed as a primary component of the microbicidal oxidase system of phagocytes. p22phox is a prerequisite for TNF-induced but not for Toll-like-receptor-induced ROS production. Mutations in this gene are associated with autosomal recessive chronic granulomatous disease (CGD) that is characterized by the failure of activated phagocytes to generate superoxide, which is important for the microbicidal activity of these cells.