Comparison of present results to those of the NMR spectroscopy studies of Kingston et al. (2004a) suggest that the 11 residues undergoing the random coil to a-helix transition are most likely located within the 486-503 region. In order to assess whether the additional N TAIL residues undergoing a less pronounced displacement upon binding to XD are located within Box3, we recorded a HSQC spectrum of 15 N uniformly labeled N TAILD3 either alone or with a twofold molar excess of XD. As shown in Figure 7 , the same peaks undergoing the large displacement in the N TAILHN -XD complex are also observed in the N TAILD3 -XD complex. In particular, among these peaks, the occurrence in both complexes of the 11 peaks that undergo the random coil to a-helix transition (see Fig. 7 , stars), provides further support that the helical folding occurs within Box2. On the other hand, the seven peaks that undergo a less dramatic displacement upon complex formation with XD (see Fig. 7 , diamonds) are not present in the spectrum recorded on the N TAILD3 -XD complex. This latter observation indicates that these seven peaks correspond to residues that are located within Box3.