X-linked chronic granulomatous disease (X-CGD) is a rare inherited immune system disorder that affects boys. X-CGD is caused by an error in a gene called gp91-phox, which affects the proper function of phagocytes — white blood cells that ordinarily help the body fight and protect against infection. Phagocytes need an enzyme called phagocyte NADPH oxidase (PHOX) to produce a chemical "bleach" that destroys bacteria and fungi, Since CGD phagocytes are missing key proteins, they fail to generate these infection-fighting chemicals.