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Structures of U1A, S6, and AdA 2H, indicating in blue the regions where folding starts according to φ‡ analysis of the transition state ensemble. It appears that the split β–α–β fold contains two nucleation sites in connection with either helix. U1A nucleates mainly in helix 1 but shows also a secondary nucleation in helix 2. S6 nucleates diffusely in both sites (parallel pathways?), whereas AdA 2H nucleates in helix 2.
