KLRK1 and neoplasm: It has been confirmed that the mechanism underlying tumor recognition by CIK cells were mainly through the interaction of NKG2D with its ligands on the tumor cells, whereas other activating natural cytotoxicity receptors, NKp30, NKp44, NKp46 and killer inhibitory receptors NKG2A and CD94 were rarely involved in the CIK-induced cytolysis effects [10, 35].