In order to further characterize the contribution of Box3 to binding, we have used fluorescence spectroscopy. Accordingly, we have designed an N-terminally hexahistidine-tagged N TAIL variant form bearing a tyrosine to tryptophan substitution at position 518 (see also Fig. 3A ). Introduction of a tryptophan residue in Box3 allowed binding events to be followed by fluorescence spectroscopy, while maximizing the conservative nature of the substitution (note that neither N TAIL nor XD contain any tryptophan residue).