BCAFs can produce a variety of MMPs that induce invasion, migration and metastasis of breast cancer cells [39], and HGF antagonists have been shown to downregulate MMP-9 activity in lung cancer cells [40], leading us to speculate that HGF may play an important role in the secretion of MMPs by BCAFs to directly or indirectly induce invasion and metastasis of breast cancer. The gene discussed is MMP9; the disease is breast cancer.