In addition to supporting colonization of cancer cells, lung-infiltrating neutrophils has also been reported to directly promote cancer proliferation via release of high levels of S100A8, S100A9, Bv8, MMP-9 and the lipid leukotriene B4, which stimulate the migration and proliferation of BC cells, and activate the MAPK/Erk pathway in BC cells to potentiate their tumorigenic capacity (6, 138). This evidence concerns the gene MMP9 and breast cancer.