MDA-MB-231 cells are an established model for malignant human breast cancer cell invasion in vitro, and it has been shown that multiple MMPs, including MMP-1, MMP-2, MMP-7, MMP-9, MMP-11, MMP-12, MMP-13, MMP-14 and MMP-17, enhance cell invasiveness [37-39]. The gene discussed is MMP9; the disease is breast cancer.