That complex formation with XD involves both Box2 and Box3, and that only Box2 undergoes a gain of a-helicity, is confirmed by NMR studies. Heteronuclear NMR experiments show that upon complex formation with XD, 11 N TAIL residues, belonging to Box2, undergo a random coil to a-helix transition, while at least seven additional residues undergo a shift in their chemical environment not accompanied by the gain of regular secondary structure elements. Our data show that these latter peak displacements correspond to residues belonging to Box3, as indicated by the absence of such peaks in both the spectra of N TAILD3 alone and in complex with XD. Thus, the NMR studies, beyond confirming the role of Box3 in the interaction with XD, also highlight that binding of N TAIL to XD implies two types of interaction, where gain of regular secondary structure is restricted to only one of the binding determinants, Box2.