ERBB2 and breast cancer: For example, trastuzumab deruxtecan shows up to an order of magnitude less potency in human HER2-expressing mouse cells compared to a human breast cancer cell line [figure S4 of (7)], an EMT6–human HER2 (hHER2) mouse model is resistant to trastuzumab emtansine (11), and large doses of trastuzumab emtansine are needed in the HER2 Fo5 model (12).