MMP9 and breast cancer: The invasion of tumor cells depends on matrix metalloproteinases (MMPs), which destroy the extracellular matrix.400 For example, DDR1 promotes invasion in MDA-MB-231 BC cells by boosting MMP-2 and MMP-9 secretion.401 By increasing MMP-2 levels, DDR1 expression can also cause colon cancer cell invasion.402 Recent studies indicate that suppressing DDR2 can decrease B16BL6 melanoma cell invasion by reducing MMP-2 and MMP-9 expression via the ERK/NF-κB pathway.403 In contrast, B16-F10 murine melanoma cells with lower DDR2 expression did not affect lung metastasis.404