Overview of the results of the experiment. A) Pfp/rag2 mice with perforin-deficient NK cells reached termination criteria earlier than rag2 mice due to amplified primary tumor growth and development of ten times more spontaneous lung metastasis (p = 0.0004). B) Primary tumors with comparable weight developed after a mean growth period of 69.4 days in rag2 mice compared to only 49.9 days mean growth period in pfp/rag2 mice (p = 0.0058). C) The mean weight of primary tumors was 1.16 g in rag mice and 1.23 g in pfp/rag2 mice, thus being statistically not different (p = 0.741). D) The mean number of circulating tumor cells detected in murine blood using qRT-PCR was 24 cells per ml in rag2 mice in contrast to 68 cells per ml in pfp/rag2 mice (p = 0.0004). E) Only 25% of rag2 mice developed spontaneous lung metastases while 81% of pfp/rag2 mice exhibited spontaneous lung metastases. Perforin-dependent killing inhibited metastatic spread relatively by 56% and by 75% in total (p = 0.002). F) In rag2 the mean number of metastases was 210 (n = 5) compared to 789 metastases in pfp/rag2 (n = 13) (p = 0.0629). G) The total amount of spontaneous lung metastases in rag2 mice was 1048 compared to 10251 in pfp/rag2 mice (p = 0.0001). Decreased NK cell cytotoxicity led to the development of ten times more spontaneous lung metastases. H) In pfp/rag2 3.9% of the tumor cells were in a mitotic state compared to 2.9% of the tumor cells in rag2 (p = 0.0107). Error bars display the standard error of the mean (SEM). Asterisks represent significant differences between both groups (* - p ≤0.05, ** - p ≤0.01 and *** - p ≤0.001). A Mann–Whitney U test was used to calculate statistical significances between both samples.