Onconase, a member of the ribonuclease superfamily, is a potent cytotoxicagent that isundergoingphase II/IIIhuman clinical trials as an antitumor drug. Native onconase from Rana pipiensand its amphibian homologs have an N-ter-minal pyroglutamyl residue that is essential for obtaining fully active enzymes with their full potential as cytotoxins. When expressed cytosolically in bacteria, Onconase is isolated with an additional methionyl (Met1) residue and glutaminyl insteadof a pyroglutamyl residue at position 1 of the N-terminus and is consequently inactivated
