Natural killer (NK) cells are a subset of white blood cells and are part of the innate immune system. Their activation, unlike that of the adaptive immune system, does not require exposure to a foreign substance. NK cells are considered a first line of immune defense in the body, as they can recognize and destroy altered cells such as virus-infected or tumour cells. On the surface of normal cells there are receptor molecules called MHC class-I, which are recognized by receptors on the surface of NK cells. The interaction of NK receptors and MHC class-I prevents NK cells from destroying normal cells. NK cells are able to destroy virus-infected cells and cancer cells because in these cells, MHC class-I molecules are often not expressed (shut off). In both human and mouse, the repertoire of receptors varies among different NK cells. To better understand how NK genes are regulated, Arefeh Rouhi is studying the mechanisms that control these variations among NK cells. Ultimately, this knowledge may lead to ways to use the body’s own immune system to protect against infections and malignancy.