N-terminal insertion peptide of HBV/G (Fig. 3b) . It is tempting to speculate that the 12 amino acid N-terminal insertion in HBV/G could be responsible for partially reducing the efficient envelopment of HBV/G, because of its proximity to several of these key residues that make contacts with the cytoplasmic domain of HBsAg. In particular, the N-terminal insertion partially covers several of these residues that were shown to be involved in secretion of mature particles, notably S17, F18, L60 and L95 (Fig. 3b) and thus might sterically hinder interactions between the L protein and core particle of HBV/G during envelopment. In addition, in previously published work, Hui et al. [20] showed that an insertion of 23 amino acids engineered on the N-terminal end of the HBV core protein could prevent core envelopment by membranes containing HBsAg. The additional N-terminal residues in HBV/G could interfere with one or more of the essential contacts that are important for core envelopment, leading to attenuated or partially deficient virion maturation. Our data show that the N-terminal insertion in the core of HBV/G partially covers the Ôhydrophobic pocketÕ which contains the residues that are essential for normal virion secretion [22] . This pocket has been shown to undergo structural changes after envelopment, which may act as a trigger, to ensure that envelopment does not take place until after DNA second-strand synthesis has completed [22] . In HBV/G the N-terminal insertion might therefore also interfere with these structural changes, as well as sterically hindering key amino acids that are thought to interact with the envelope. A reduced ability for virion envelopment and secretion might partially explain why the presence of HBV/G in patients is strongly associated with co-infection by another genotype.