The folding of FADD DD can be compared with that of the all-beta Greek key Ig-like domains studied earlier, including TNfn3,11,45 FNfn10,13 CAfn2,14 and TI I279,10 in our laboratory, and CD2d1.15 In the Ig-like domains, tertiary structure is the dominant factor influencing the folding mechanism. These domains exhibit a near-classical nucleation-condensation mechanism where long-range key residues, in the central BCEF strands, interact in the TS to set up the complicated topology; the transition state is an expanded version of the native state, with the folding nucleus involving secondary and tertiary interactions, centred around the structural core. Peripheral regions pack late.