Expression of MMPs in different human breast cancer cell lines analyzed by Western blot. Protein lysates were isolated from the four breast cancer cell lines and separated by polyacrylamid gel electrophoresis. MMP proteins were visualized using specific antibodies, capable of recognizing both, the inactive and active forms of MMPs (arrows inactive/active). β-actin was used as an internal loading control. Protein amounts were quantified using the Bradford method and normalized to the amount of β-actin for each sample.