One of the major questions in malaria vaccine development is what assay can serve as a surrogate for clinical protection. At this stage, there is no immunological assay proven to be correlated with clinical protection. As noted above, anti-AMA1 antibodies induced both by a malaria infection [26, 27] and by an AMA1 immunization in malaria naïve individuals [14, 16, 19, [21] [22] [23] show growth-inhibitory activity, and a recent clinical challenge trial with AMA1 vaccine in a malaria naïve population has shown that there is a significant correlation between in vivo parasite multiplicationrate and growth-inhibitory activity measured by in vitro GIA in vaccine recipients (our unpublished observation). In addition, some epidemiological studies have shown that the total growthinhibitory activity (or invasion-inhibition activity) before the malaria transmission season is significantly associated with a subsequent malaria risk [46, 47] . However, other epidemiological studies have not shown such associations [48, 49] . Therefore, one may dispute the usage of this assay for vaccine development. These epidemiological studies did not test the specificity of antibodies. In addition, interpretation of growth-inhibitory activity of samples from epidemiological studies for a specific antigen is not straightforward. As shown in Figure 1 , when anti-AMA1-specific IgGs were separated from total IgG pools of Malian children, they showed similar activity as anti-AMA1 IgG from U.S. vaccinees. However, the AMA1-depleted IgG, i.e., non-AMA1 IgG, displayed the same level of inhibition as the original total IgG pool ( Figure 2 ). We also observed the same phenomenon in our previous study [27] where we separated non-AMA1 IgG from Malian adults' total IgGs. In addition, another of our studies [28] showed that pre-incubation of Malian children's total IgGs with AMA1 protein did not reduce growth-inhibitory activity induced by natural infection (even though the total IgGs had higher level of AMA1 titer), while the pre-incubation did diminish vaccineinduced activity almost completely. All of the data indicate that the overall growth-inhibitory activity induced by a natural infection is not simply the sum of growth-inhibitory activity of individual antibodies. Our preliminary study shows that there is no additive effect of growth-inhibitory activity between rabbit anti-AMA1 antibody and anti-MSP1 antibody (our unpublished observation). Furthermore, it is possible that malaria infection induces not only growth-inhibitory antibodies, but also interfering antibodies in humans, at least in the case of AMA1, as shown in this study. While it is difficult to prove whether such non-additive (and/or interference) effects really occur in vivo, or whether this is just a limitation of the in vitro assay, in either case the growth-inhibitory activity of antibodies from a malaria endemic area for a specific antigen should be interpreted with caution. However, the GIA is the only functional assay widely used for AMA1-based vaccine development. If the growth-inhibitory activity measured by the GIA reflects some mechanism of protection in vivo, the results of this study suggest that pre-existing anti-malaria immunity may modulate the efficacy of the AMA1 vaccine. At a minimum, we believe it is extremely important to take these findings into account in evaluating immunogenicity of AMA1-based vaccines when a study is conducted in populations exposed to malaria. Hepatitis B Virus Genotype G forms core-like particles with unique structural properties SUMMARY: We have determined the structure of the core capsid of an unusual variant of hepatitis B virus, genotype G (HBV/G) at 14 Å resolution, using cryo-electron microscopy. The structure reveals surface features not present in the prototype HBV/A genotype. HBV/G is novel in that it has a unique 36- bp insertion downstream of the core gene start codon. This results in a twelve amino acid insertion at the N-terminal end of the core protein, and two stop codons in the precore region that prevent the expression of HBeAg. HBV/G replication in patients is associated with co-infection with another genotype of HBV, suggesting that HBV/G may have reduced replication efficiency in vivo. We localized the N-terminal insertion in HBV/G and show that it forms two additional masses on the core surface adjacent to each of the dimer-spikes and have modelled the structure of the additional residues within this density. We show that the position of the insertion would not interfere with translocation of nucleic acids through the pores to the core interior compartment. However, the insertion may partially obscure several residues on the core surface that are known to play a role in envelopment and secretion of virions, or that could affect structural rearrangements that may trigger envelopment after DNA second-strand synthesis. Hepatitis B virus (HBV) is one of the most important causes of liver disease in humans. Although HBV infection is preventable by vaccination, it currently affects more than 350 million people worldwide, with many infected as long-term carriers [1] . HBV is a member of the Hepadnaviridae with a partially double stranded DNA genome of 3.2 kb in size [2] . The HBV nucleocapsid core is icosahedral and enveloped by a membrane containing the surface antigen glycoprotein (HBsAg). At the structural level, the nucleocapsid of the prototype HBV (genotype A) has been studied extensively by both cryo-electron microscopy (cryo-EM) and X-ray crystallography [2] [3] [4] . In addition to the presence of HBsAg, two other diagnostic markers can frequently be detected in the serum of infected patients: antibody directed against the core antigen (HBcAg, consisting of assembled capsids) and the e antigen (HBeAg) an alternate translation product of the core gene that is cleaved and secreted posttranslationally. The HBcAg is dimeric, and assembled cores exhibit oligomeric dimorphism: a proportion has T = 3 symmetry (with 180 subunits, consisting of 90 dimers of HBcAg) and the other proportion having T = 4 symmetry (240 subunits and 120 dimers) [5] . This icosahedral dimorphism is present both in cores that are purified from the serum of HBV-infected patients and in core-like particles which self-assemble upon in vitro expression of the core protein. When the HBV virion enters cells, the uncoated core is transported to the nucleus where HBV genomic DNA is repaired to the covalently closed circular DNA form. During replication, the RNA pregenome is incorporated into assembling cores, and reverse transcription to form the genomic DNA occurs within the assembled core [2] . The core protein dimers form spike-like protrusions which extend from the spherical surface of the core particle, and also form a network of pores or holes [2] .