Onconase is an extremely stable member of the RNase A superfamily. The increase in the thermodynamic stability by 20 kJÆmol )1 in comparison to RNase A was expected to result in altered folding behavior. Despite the lack of cis-Pro residues in native Onconase, refolding at low concentrations of guanidine hydrochloride was complex and showed three kinetic phases (fast, medium, and slow), with rate constants differing by a factor of about 10 each.