In order to further explore the nature of the interaction established between Box3 and XD, we have used NMR spectroscopy. To this endeavor, we have recorded a HSQC spectrum of 15 N uniformly labeled N TAILHN either alone or in the presence of a twofold molar excess of XD, as well as of 15 N uniformly labeled N TAILD3 in the same conditions. This analysis allowed a quantitative estimation of the number of residues involved in the interaction with XD by following chemical shift changes in the backbone amide and proton resonances upon addition of unlabeled XD. Upon addition of XD to N TAILHN , 16 correlation peaks are displaced. Among them, 11 undergo an upfield shift (see Fig. 7 , stars), which indicates a random coil to a-helix transition, and two correspond to the side chain of either a Gln or an Asn. Additionally, at least seven additional peaks undergo a less dramatic displacement (see Fig. 7 , diamonds). Because of its small amplitude, this shift most likely does not reflect a conformational change. Nevertheless, it is indicative of a change in the chemical environment of these residues resulting either by direct interaction with the partner or by local magnetic perturbations of residues spatially close to the newly formed a-helix. Based on this experiment, however, the determination of N TAIL residues involved in these two types of shifts is not possible. The precise identification of these residues would require the full assignment of the N TAIL HSQC spectrum, which is complicated by the strong signal overlapping inherent in the predominantly unfolded nature of N TAIL .