Is there a single pathway for the folding of a polypeptide chain?

We argue that folding of the compact domains of proteins can occur with adequate rapidity in the absence of a unique directed mechanism, provided that native-like local structure dominates the folding process. We further suggest that the evolution of amino acid sequences should favor multiple paths to the folded state. Existing physicochemical and mutational data are not inconsistent with a many-pathway model. The analogy of a jigsaw puzzle, with multiple routes to a unique solution, appears to be particularly apt.