Beforehand, we checked whether XD possesses the same structure in solution as in the crystal. SAXS experiments performed on XD showed that it has a globular shape with a radius of gyration (R g ), extrapolated at a nil concentration, of 12.1 6 0.8 Å and a maximum diameter D max of 41 6 1 Å (data not shown). Comparison with the crystal structure using the program CRYSOL (Svergun et al. 1995) indicates that the scattering profile calculated for the crystal structure (PDB code 1OKS) is identical to the experimental one, fitting the data with a 2 of 1.4 and giving a theoretical R g of 12.3 Å (data not shown). These results suggest that the overall conformation of XD in solution is similar to that observed in the crystal.