Is the pathway of protein folding determined by the relative stability of folding intermediates, or by the relative height of the activation barriers leading to these intermediates? This is a fundamental question for resolving the Levinthal paradox, which stated that protein folding by a random search mechanism would require a time too long to be plausible. From a thermodynamic point of view, protein folding is a spontaneous process; an unfolded protein can refold to the native structure because it is the structure at the global free energy minimum (1, 2). However, the thermodynamic hypothesis of protein folding does not address the question on rates