HBx is aptly named as its entire function and cellular localization remain a mystery. This 154 amino acid protein has conserved sequences between human, woodchuck, and ground squirrel hepadnaviruses. However, avian hepadnaviruses lack HBx. As such, HBx is not believed to be involved directly in genomic replication or viral assembly. HBx has not been found in mature virions nor associated with nucleocapsid particles.
Sequence analysis suggests that HBx is localized in the cytoplasm.
HBx can be phosphorylated. However, studies on HBx have been hampered by its apparent rapid turnover when expressed in vitro . A plethora of functions have been assigned to HBx. Overexpression of HBx in hepatocyte cultures and transgenic mice results in a tumorigenic phenotype. Though many functions have been associated with HBx, their validity remains in question simply due to the levels of HBx seen in infected cells.