MMP9 and breast cancer: Pulmonary metastasis was also associated with progressive degradation of elastin fibers (Fig. 2a–g) and an increase in the expression of metalloproteinase 2, 9, and 14 (MMP-2, MMP-9, MMP-14) in the lungs 3 and 4 weeks after breast cancer cell injection, respectively (Fig. 2h), being compatible with tissue remodeling accompanying the advanced stage of metastasis progression.