The involvement of additional N TAIL regions other than the a-MoRE in binding to XD is confirmed by surface plasmon resonance studies, where the contribution of Box3 to XD binding can be quantitatively estimated. Removal of either Box3 alone or Box2 plus Box3 leads to a strong decrease (three orders of magnitude) in the affinity as compared to full-length N TAIL , thus indicating that Box3 contribution to binding is similar to that of Box2.