To this end, endocytosis assay for Zapadcine-1 was carried out and showed that FITC-labeled Zapadcine-1 (green) on DR5-expressing MSTO-211H cells could be internalized rapidly into the lysosome tracking with lysosome specific Lyso-Tracker red dye (Figure 2B), suggesting that lysozyme proteases, such as tissue protease B (cathepsin B) and plasma enzyme (plasmin), could be able to break the amide bond between Val-Cit and aniline in the linker of Zapadcine-1, and therefore result in the self-elimination of the p-aminobenzyl carbonyl to release MMAD killing tumor cells 37, 38. Here, CTSB is linked to neoplasm.