Critical component of the membrane-bound oxidase of phagocytes that generates superoxide. It is the terminal component of a respiratory chain that transfers single electrons from cytoplasmic NADPH across the plasma membrane to molecular oxygen on the exterior. Also functions as a voltage-gated proton channel that mediates the H(+) currents of resting phagocytes. It participates in the regulation of cellular pH and is blocked by zinc. Defects in CYBB are a cause of granulomatous disease,chronic, X-linked (CGD). A disorder characterized by the inability of neutrophils and phagocytes to kill microbes that they have ingested. Patients suffer from life- threatening bacterial/fungal infections. Defects in CYBB are a cause of mycobacteriosis atypical X-linked type 2 (AMCBX2). A rare condition characterized by predisposition to illness caused by moderately virulent mycobacterial species, such as Bacillus Calmette-Guerin (BCG) vaccine and environmental non-tuberculous mycobacteria, and by the more virulent Mycobacterium tuberculosis. Other microorganisms rarely cause severe clinical disease in individuals with susceptibility to mycobacterial infections. Note: This description may include information from UniProtKB.