MMP1 gene expression in PBMC from controls was significantly increased after co-cultures with breast cancer cell lines (MCF-7 and MDA-MB-231), and, in turn, PBMC induced an increased proliferative capacity of the mildly aggressive breast cancer cell line MCF-7, especially PBMC from breast cancer patients, and a greater invasive capacity of the highly invasive MDA-MB-231 cell line. Here, MMP1 is linked to breast cancer.