Based on the analysis of tumor-infiltrating NK cells in mice bearing melanomas, we revealed that CTLA4Ig could significantly promote the cytolytic activity of the infiltrating NK cells in vivo (Figure 3A) via the up-regulation of NK cell cytotoxicity (Figure 3C) but not through the production of IFN-γ and TNF-α (Figure 3B). Here, TNF is linked to neoplasm.