We next investigated lung metastasis in twelve−week−old animals and observed the appearance of a higher number of metastases in PyMT−Cxcr2−/− compared to PyMT WT animals (Figure 3C), demonstrating that Cxcr2 knock−down affects not only primary tumor growth and spleen size, but also distant metastasis. This evidence concerns the gene CXCR2 and neoplasm.