NPEPPS and neoplasm: The Leu-AMC cleavage assay described above found that tumor cells isolated from NPEPPS−/−(EV) and NPEPPS−/−(E353V-FLAG) showed lower catalytic activity compared to those from NPEPPS−/−(WT-FLAG) tumors, supporting our prior findings that NPEPPS catalytic activity correlates with treatment response (Fig. 5C).