The presence of different genotypes of HBV has been correlated with different geographical regions and with varying severity of disease [6] . Recently, a novel genotype of HBV (HBV/G) was described [7] , which has a unique 36-bp insertion downstream of the core gene start codon, resulting in a twelve amino acid insertion at the N-terminal end of the core protein. In addition, HBV/G has two stop codons in the precore region at positions 2 and 28, which prevent the expression of HBeAg [8] . The fact that HBV/G is almost exclusively detected in patients in association with one of the other seven HBV genotypes (usually HBV/A), and the frequent presence of HBeAg in HBV/G-infected individuals, suggests that HBV/G replication requires co-infection with a helper virus of another HBV genotype [9, 10] . The unique epidemiological and clinical profile of HBV/G suggested that it could be deficient in some other aspects of its function, perhaps as a result of the differences in the capsid structure and function caused by the presence of the N-terminal insertion in the core protein. Although HBV/G has been shown to be capable of replication in the absence of another co-infecting genotype in vitro, the envelopment of genotype G appeared to be less efficient than that of genotype A, while the presence of the 36-nt insertion enhanced core protein expression and genome replication [9] . The aim of this study was to investigate for the first time the structural morphology of the HBV/G core particles in comparison with HBV/A, and in particular to localize the 12 amino acid insertion. We used cryo-electron microscopy and structural modelling to investigate structural differences in the features of HBV/G core-like particles that could influence functions such as replication, genome packaging, or possibly HBsAg envelopment of the capsid and viral secretion.