Our findings are very similar to the observations made by Sodeur et al.[22] where most notably the total number of metastases of all cell lines used in their experiment were considerably higher in pfp/rag2 mice than in scid mice which leaves hardly any doubt that perforin-dependent direct cytotoxicity of NK cells plays a crucial role in the anti-cancer control of our immune system indicating that the presence of NK cells with the abilitiy of direct perforin-dependent killing is more important in determining the number of metastases than the mouse strain background. The gene discussed is PRF1; the disease is cancer.