ESR1 and breast cancer: This work aimed to investigate and elucidate the metastatic potential of cancer cells by evaluating the cell stiffness, the spread and the adhesion strength of two breast cancer cell lines with different phenotypes, MCF-7/ER+ and SKBR-3/HER2+, prior to and post-treatment with the antitumor agents tamoxifen, a selective estrogen receptor modulator; and trastuzumab and pertuzumab, monoclonal antibodies against HER2.