Resolution of the structure was estimated by plotting the Fourier shell correlation (FSC) against resolution, giving a value of 14 Å at an FSC of 0.5. The resolution of the HBV/A model 1QGT.pdb determined by X-ray crystallography [4] was then reduced to 14 Å using the EMAN program Ôpdb2mrcÕ [13] . Both maps were normalized for electron density, aligned and compared by creating a difference map. The contour level of the difference map was selected so that its mass matched the volume of the additional 12 residue N-terminal insert of the HBV/G core structure [17] . The tertiary structure of the 12 residues was predicted by the I-TASSER server [18] , and the resulting coordinates were modelled in to the difference map density HBV/G structure using the program COOT [19] .