In order to directly measure the contribution of the different N TAIL boxes to binding, we have studied binding reactions between XD and N TAIL deletion proteins. Changes in surface plasmon resonance were monitored in real time as the N TAIL proteins passed over sensor chips to which XD was covalently coupled. This analytical approach is ideally suited to study reversible, lowaffinity protein-protein interactions that typify interactions involving intrinsically disordered proteins (Wright and Dyson 1999; Dunker and Obradovic 2001; Uversky 2002) . Moreover, reaction rate and equilibrium constants were calculated for reactions with different protein substrates, allowing differences in XD binding affinities to be readily quantified.