Contribution of Box3 to binding without dramatic structural change is supported by fluorescence spectroscopy data, which show an increase in the fluorescence intensity of N TAILW518 upon addition of XD. Increases in the fluorescence intensity upon binding to a partner/ ligand have been already documented, for both folded (Bette et al. 2002) and intrinsically unstructured proteins (Raggett et al. 1998) , and indicate that the chemical environment of the Trp, although remaining mostly polar, is changed as a result of the addition of the partner. However, the fluorescence data do not enable us to discriminate between a direct Trp 518-XD interaction and secondary effects resulting from local perturbations triggered by a direct Box2-XD interaction.