Among all deletions in the entire preS region, truncations in preS2 were the most common in our investigation, suggesting that the preS2 region may be selectively affected by immune pressure. Further studies are needed to clarify the role played by the host immune response in inducing deletions in preS1 and preS2 genes. Another interesting phenomenon is the high rate of deletions in the 5′ terminus of preS1 in

our samples compared to immune-suppressed subjects as shown in Figure 2A. Although it does not encode any known epitope, this region spans OICR-9429 the host determining region which contributes to the species specificity of HBV [24]. Interestingly, genotype D of HBV, which is 11 amino acids shorter than that of genotype B, does not contain this region and resembles the 5′ terminus of the preS1 deletion mutant [25]. PreS2 deletions may promote HBV immune escape after recovery of host immune function following antiviral treatment Deletions have been shown to confer resistance to lamivudine (LMV) in an HIV-related study, and certain deletion mutants of HBV were shown to be insensitive to LMV [26, DNA Damage inhibitor 27]. In our study, we observed the accumulation of preS deletions correlating to antiviral therapy. However, our in vitro experiments demonstrated

that the HBV with preS deletion alone did not confer resistance to antiviral therapy in such mutants, similar to a recent observation by Ohkawa et al. [28]. This inconsistency between the epidemiological statistics and in vitro experiments is perhaps not surprising when the most common feature of HBV infection, the existence of quasispecies within MG-132 cell line an individual,

is considered. Despite very complex patterns of HBV quasispecies, which were resolved by clone sequencing or high throughput sequencing, we, along with others, have observed that the wild type never disappears from the viral composition. For instance, our recent pyrosequencing study on HBV quasispecies showed that the lowest proportion of the wt strain in patients was around 1% (Zhang et al., unpublished). These data strongly suggest the coordination of wt and various types of mutants which may not survive by themselves alone but whose presence may be beneficial to the viral population in vivo. Such coordination between viral strains may well explain our results. Generally speaking, antiviral therapy would also result in the recovery or enhancement of the host defense system, which in turn would increase the selection pressure on mutants, such as preS deletions, that may promote immune escape. Supporting evidence also stems from research that suggests an improvement in CTL responsiveness to HBV in CH patients following LMV treatment [29].