ERBB2 and neoplasm: The proteolytically activated, unmasked HER2-TCE demonstrated highly potent cytotoxicity against high HER2-expressing tumor cells, SKOV3 (∼650,000 HER2 receptors per cell; Fig. 2b) and BT-474 (∼975,000 HER2 receptors per cell; Fig. 2c), showing nearly complete target cell killing with 50% effective concentrations (EC50) in the single-digit picomolar range.