We have studied the folding of two protein families extensively. The first is the complex, all-beta Greek key Ig-like fold.9–11,13–15 Topology is the dominant factor influencing the folding of Ig-like domains. All these proteins fold via a nucleation condensation process, where key long-range interactions, between residues in the central BCEF strands, form early to nucleate folding. The transition state of these proteins is an expanded form of the native state and local interactions are not involved in determining the folding pathway.