The exact mechanisms underlying tumor cell recognition and elimination have not been fully elucidated in CIK cells; however, the NK cell-activating receptor NK group 2 member D (NKG2D), which is expressed on the membrane of CIK cells and interacts with MHC-unrestricted ligands on tumor cells, may play a predominant role (9). The gene discussed is KLRK1; the disease is neoplasm.