The experiments using perforin knockout mice, adoptive transfer of perforin-deficient hepatic NK cells and the demonstration of direct cytotoxicity of B16 melanoma by the immature NK cell subsets clearly indicate that the CD27+ CD11b– immature subset in the liver contains cytotoxic cells and that it is their cytotoxicity that is protective against liver metastasis. Here, PRF1 is linked to melanoma.