The low-resolution shape of the N TAILHN -XD complex was determined ab initio from the scattering curve using the program GASBOR ). This program restores lowresolution shapes of protein and calculates a volume filled with densely packed spheres (dummy residues) fitting the experimental scattering curve by a simulated annealing minimization procedure and considering the protein as an assembly of dummy residues centered on the C a positions (spheres of 3.8 Å diameter) with a nearest-neighbour distribution constraint. Several independent fits were run with no symmetry restriction with an input of 188 dummy residues, corresponding to the 56 residues of X D and 132 residues of N TAILHN . The program package CREDO (CHADD and GLOOPY) was used to restore the low resolution model of N TAILHN in complex with XD, with the crystal structure of the chimeric protein between XD and the region of N TAIL encompassing residues 486-505 (XD-N TAIL486-505 ) (Kingston et al. 2004a ) as template. CREDO is an extension of the original program GASBOR. It calculates the structure of missing domains or loops of crystal structures from the experimental scattering curve of the entire particle and represents them by an ensemble of dummy residues forming a chain-compatible model.