In addition to the number of spontaneous metastases the morphology of the spontaneous lung metastases also differed considerably: metastases in rag2 mice were only 1–10 tumor cells in line with single cell metastases (DTCs) being the most frequent detected metastatic deposits while metastases in pfp/rag2 mice generally consisted of 10–100 tumor cells on average presumably showing that metastasis cell killing also takes place in the metastatic organ. Here, PRF1 is linked to neoplasm.