CIK cells’ anti-tumor activity is perforin mediated and was mainly attributed to the cell-cell contact-dependent natural killer group 2 member D (NKG2D) cell-surface receptor, since antibody blocking experiments using anti-NKG2D antibody or siRNA showed that CIK cells mainly lost their T-cell receptor (TCR)-independent antitumor cytotoxicity against malignant cells. This evidence concerns the gene KLRK1 and neoplasm.