Rothwell et al. reported in an orthotopic Rag1-/- murine xenograft model of HER2-positive BM from BC that a single prophylactic intrathecal administration of an AAV9-trastuzumab vector increased the median OS (124 versus 50 days), attenuated brain tumor growth and preserved both the HER2 antigen specificity and the natural killer cell–associated mechanism of action of trastuzumab. The gene discussed is ERBB2; the disease is breast cancer.