The hepadnavirus (1–3) family includes hepatitis B virus (HBV), woodchuck hepatitis virus (WHV) (4), ground squirrel hepatitis virus (GSHV) (5) and duck hepatitis B virus (DHBV) (6). These viruses share unique ultrastructural, molecular and biological features. HBV has great medical importance in many parts of the world. More important numerically than acute hepatitis B in high prevalence geographic regions is the liver disease associated with chronic infection. There appear to be more than 200 million chronically infected humans in the world (7), and these HBV infections appear to be the single most common cause of chronic liver disease and liver cancer in man (7, 8). All hepadnaviruses share the propensity for silent infection in early life leading to persistence of the virus, and hepatocellular carcinoma (HCC) is clearly associated with longstanding persistent infection in man (7, 8), woodchucks (1, 9, 10) and ground squirrels (10a). Although the viral DNA has been found to be integrated in cellular DNA of many HCC in man (11), woodchucks (9, 10) and ground squirrels (10a), the precise role of the virus in tumor formation has not been defined.
Hepadna viruses have an interesting molecular structure and mechanisms of replication, and they appear to share certain important features with retroviruses as reviewed here. It is of interest to define similarities and differences between hepadnaviruses and retroviruses in order to understand their evolutionary relationship and to determine whether they share a common oncogenic mechanism, since infection with members of both virus families is associated with neoplastic disease.