Mice housed in an enriched environment display a remarkable antitumor phenotype. Enriching the housing conditions for mice enhances the tumor infiltration and the cytotoxicity ability of natural killer (NK) cells via sympathetic nerve-dependent regulation of NKG2D and CCR5 expressions on NK cells. The immunohistochemical staining for NK1.1 shows an increased number of infiltrating NK cells in transplanted tumors from enriched environment-housed mice. For details, see article by Song and colleagues on page 1611.