The far-UV circular dichroism (CD) spectra of N TAIL deletion proteins at neutral pH are typical of unstructured proteins, as seen by their large negative ellipticity at 198 nm and very low ellipticity at 185 nm ( Fig. 4A -C). The solvent 2,2,2-trifluoroethanol (TFE) mimics the hydrophobic environment experienced by proteins in protein-protein interactions, and is therefore widely used as a probe to unveil disordered regions having a propensity to undergo an induced folding (Hua et al. 1998 ). Thus, we have recorded CD spectra of N TAIL deletion proteins in the presence of increasing concentrations of TFE ( Fig. 4A -C). All proteins show an increasing gain of a-helicity upon addition of TFE, as indicated by the characteristic maximum at 190 nm and minima at 208 nm and 222 nm ( Fig. 4A -C). In the case of N TAILD3 and N TAILD1 , most unstructured-to-structured transitions take place in the presence of 20% TFE, a concentration at which the a-helical content is estimated to be about 13% (using the ellipticity at 222 nm) (Fig.  4D ). On the other hand, in the case of N TAILD2,3 , TFE concentrations as high as 30% are required for most pronounced unstructured-to-structured transitions to take place (see Fig. 4B ). Moreover, in the presence of 30% TFE, the a-helical content of N TAILD2,3 is not only lower (11%) than that (16%) of N TAILD1 and N TAILD3 , but also lower than the a-helical content observed at 20% TFE for the two other N TAIL deletion proteins (Fig. 4D) . Therefore, N TAILD2,3 displays the lowest a-helical potential, while N TAILD1 and N TAILD3 exhibit a folding propensity similar to that observed for N TAIL Bourhis et al. 2004 ). These results, beyond confirming that the region spanning residues 489-516 affects the folding potential of N TAIL (Bourhis et al. 2004) , suggest that neither Box1 nor Box3 contribute to the a-helical propensity of N TAIL , in agreement with the secondary structure prediction provided by PSI-PRED (McGuffin et al. 2000) and PHD (Rost 1996) , which both predict an a-helix (residues 489-504, see 