The present invention provides a modified toxin having an ETA moiety that has the furin site replaced with a cancer-associated protease site. The present invention also provides modified immunotoxins having a ligand that binds to a cancer cell attached to an ETA moiety that has the furin site replaced with a cancer-associated protease site. Also provided are a method of inhibiting or destroying mammalian cancer cells using the immunotoxins of the invention and pharmaceutical compositions for treating human cancer.