PRSS21 and neoplasm: Although the engineered cleavage site in PrAg-PCIS showed a preference for activation cleavage by testisin, it was also able to be cleaved in vitro by the recombinant catalytic domains of additional serine proteases, notably hepsin and matriptase, suggesting that activation of PrAg-PCIS is not completely specific to testisin, but that additional active pericellular serine proteases overexpressed by tumor cells may also be effectively targeted by this modified anthrax toxin prodrug strategy.