MMP9 and neoplasm: The detachment of invading tumor cells from the primary tumor mass accompanied by decreased expression of Cx43 and increased CD44 expression, followed by the anchored and degradation of ECM by overexpressed MMP-9 and MMP-2, allow the colonization of tumor cells into normal brain tissues such as brain parenchyma, leptomeningeal space, white matter tracts of corpus callosum, and perivascular space [19,20].