1QGT (Figs 2a,b) [4] . The difference map created shows additional mass around the base of the spikes of the HBV core protomers (Figs 2c,d) . When the difference map is superimposed with the HBV/A atomic coordinates the additional mass of the HBV/G structure is found to be adjacent to the N-terminal residues of the core protein ( Fig. 2d and Figs 3a, b) . The extra mass protrudes from the surface of the core particle and sits beside the capsid pores and not over them, so this feature would not effectively reduce the diffusive movement of nucleotide triphosphates into the core interior, or affect the release of nucleic acids from the core (Fig. 3a, b) . When modelling the extra 12 residues by TASSER/COOT, it was apparent that there are a number of possible arrangements that 12 residues can take up and still be contained within the corresponding density (Fig. 3c) . Previously, a crystal structure of an HBV capsid protein engineered with an additional 11 residue extension to its N-terminal end has been determined, PDB accession 2QIJ [21] . Although the orientation was different from that observed in HBV/G, both additional masses were pointing away from the four-helix bundle as opposed to interacting with the main body of the core protein (Fig. 3c ).