MMP9 and breast cancer: In summary, the mechanisms by which breast cancer cells traverse the BBB or BCFB involve disruption of tight junctions (e.g., via ENPP1 and MMP-9), regulation of signaling pathways (such as the circBCBM1–BRD4–SHH axis), immune-mediated processes (complement C3–C3aR), and exosome-driven transcellular transport.