A two-and threefold molar excess of XD is required to induce the most pronounced structural transitions of fulllength and truncated forms of N TAIL , respectively. In contrast, an equimolar amount of PCT is sufficient to produce the same effect Bourhis et al. 2004) . This difference can be accounted for by a lower affinity of N TAIL towards XD, compared to PCT, rather than to the formation of a 1:2 or 1:3 stoichiometric complex. Indeed, formation of a 1:1 stoichiometric complex between XD and a peptide corresponding to residues 477-505 of N, has been documented using isothermal titration calorimetry (Kingston et al. 2004b ). The higher affinity of N TAIL for PCT compared to XD could be ascribed either to cooperativity phenomena among the different XDs within the PCT tetramer (Rahaman et al. 2004) or to the possible contribution of other PCT regions to binding. This latter possibility can be ruled out based on recent data pointing out XD as the sole PCT region contributing to binding (Kingston et al. 2004b; S. Longhi and M.J. Oglesbee, unpubl.) .