The folding of the two three-helix bundles of FADD DD can be compared with the folding of other three-helix bundle proteins. A number of these have been studied extensively, using Φ-value analysis. In some cases, one well-formed helix is observed in the TS with other elements of structure packed against it (as, for example, in protein A27,57), whereas in others, two elements of structure come together with the third helix relatively unstructured (e.g. spectrin domains17,18 and peripheral subunit binding domains58). The three-helix bundles of FADD DD fall into this second category. Interestingly, as in the spectrin domains, the helices that are in contact are those that are separated in sequence (H1 with H5 and H2 with H4). This suggests that formation of these long-range interactions is the important step for folding these bundles.