Spectroscopy studies showed that Box3 contributes to binding to XD but does not undergo any gain of regular secondary structure. Although Box3 does not affect the folding potential of N TAIL , as indicated by CD studies in the presence of TFE, the removal of Box3 significantly reduces the ability of N TAIL to undergo induced folding in the presence of XD. This supports a role for Box3 in binding to XD. Further removal of Box2 results in a truncated form that has a significantly decreased folding potential and has lost the ability to undergo induced folding in the presence of XD. These results are consistent with the unique a-helical forming potential of Box2 and its role as primary binding site for XD.