CD9 and breast cancer: Moreover, flow cytometry using breast cancer cell lines with varied expression levels of EGFR indicated that BP-αCD3-αEGFR-ARC Exos along with CD9-CD38/αCD3-αEGFR Exos and αCD3-αEGFR Exos can tightly bind to EGFR-positive BT-20 and MDA-MB-231 cells, but not EGFR-negative MDA-MB-453 cells (Supplementary Fig. 9).