We speculate that hUCMSCs-LV-IL-21 elevated IFN-γ and TNF-α related to the secretion of IL-21, and that the elevated IFN-γ and TNF-α might markedly enhance the NK cytotoxicity that was supported by increase of expression of NKG2D and MIC A in the tumor tissues of the mice treated with hUCMSCs-LV-IL-21group. This evidence concerns the gene KLRK1 and neoplasm.