CD226 and neoplasm: NK cell-mediated cytotoxicity against target tumor cells requires the co-engagement of several activation receptors—such as the natural cytotoxicity receptors (NCRs) NKp46 and NKp80—or other co-stimulatory receptors—including natural killer group 2 member D (NKG2D) or DNAX accessory molecule (DNAM)-1 [1].