ESR1 and breast cancer: Using the ER+ cell line (MCF7), the TNBC cell line (MDA-MB-231), and the brain-metastatic-specific derivative of the TNBC line (MDA-MB-231BR) as mentioned above, the authors were able to show that secreted factors from TNBC cells have a greater ability to alter the BBB and aid in breast cancer brain metastatic development [7].