ERBB2 and neoplasm: We selected the affibody ZHER2:342 with a KD (equilibrium dissociation constant) of 22 pM to the human epidermal growth factor receptor-2 (HER2) [27] as the tumor-targeting ligand and prepared the small ADC molecule ZHER2-MMAE through a site-directed coupling strategy (Figure 1B) catalyzed by sortase A with the microtubule antagonist monomethyl auristatin E (MMAE).