ERBB2 and breast cancer: The three ADCs presented equal sub-nanomolar IC50’s in the five HER2-expressing cell lines (Figure 3C), i.e., SKBR-3 (IC50 = 0.18 ± 0.04 nM), NCI-N87 (IC50 = 0.20 ± 0.05 nM), MDA-MB-453 (IC50 = 0.20 ± 0.10 nM), MDA-MB-361 (IC50 = 2.0 ± 0.8 nM), and BT-474 (IC50 = 0.9 ± 0.4 nM), and no cytotoxicity against MCF-7 (IC50 > 10 nM), a HER-2 negative breast cancer cell line.