For the first time, researchers have demonstrated that the response of natural killer (NK) cells is detrimental to glioblastoma virotherapy, a novel way of treating malignant brain cancer by injecting a virus into the tumor. A number of clinical trials are currently underway to test whether glioblastoma virotherapy will facilitate antitumor efficacy, but new research shows that in pre-clinical models, NK cells are killing off the virus – infected cells, thus rendering the therapy less effective.