In terms of enzyme sensitive linkers, peptide based linker is sensitive to the lysosomal protease and have been employed in a number of ADCs.47 The lysosomal proteases, such as cathepsin B, are generally overexpressed in cancer cells, enabling the accurate drug release in the vicinity of the tumor.48 Moreover, because of the existence of protease inhibitors in the blood, the linker are normally stable in the systemic circulation and it decreases the risk of side effects.49 Among approved ADC drugs, 9 of 14 use peptide based linkers. Here, CTSB is linked to cancer.