MMP13 and invasive ductal breast carcinoma: MMP13 gene, the expression of which was induced more than 3-fold after ERβ knockdown, codes for matrix metalloproteinase 13 (collagenase 3), a protease originally identified in breast carcinoma, which is also known to promote tumor cell invasion and has been previously reported as a molecular marker for transition of ductal carcinoma in situ lesions to invasive ductal carcinomas [48].