ERBB2 and breast cancer: To evaluate the impact of Her2 surface expression on cancer cells, a crucial factor for TcE on-target/off-tumor targeting (43), we included two cell lines with differing Her2 membrane densities: BT474 ductal carcinoma breast cancer cells, with ~6 × 105 molecules Her2/cell (Her2High), and MCF-7 mammary epithelium adenocarcinoma cells, with ~2 × 104 Her2/cell (Her2Low) (44).