PRSS21 and neoplasm: Strategies employed include the reengineering of PA to bind tumor cell surface-enriched proteins (33, 34), and the reengineering of PA to be proteolytically activated by proteases enriched in the tumor microenvironment, including matrix metalloproteinases (35, 36, 37, 38, 39, 40, 41, 42), urokinase plasminogen activator (37, 39, 41, 44, 47, 53), and testisin (48).