X-linked chronic granulomatous disease (X-CGD) is a rare genetic disorder, which affects boys. It is a primary immunodeficiency disorder which results from an inability of the white blood cells called phagocytic cells (or phagocytes) to kill invading bacteria and fungi. These cells have difficulty forming the free radicals (most importantly the superoxide radical due to defective phagocyte NADPH oxidase complex) which are important in the killing of ingested pathogens. In X-CGD (which accounts for two thirds of CGD patients), the defect lies in a gene which makes up a critical part of the NADPH-oxidase complex (the catalytic subunit; gp91-phox protein). Therefore they kill bacteria and fungi poorly, and the patients suffer from severe and recurrent infections. This also results in inflammation which can damage parts of the body such as the lung and gut.