Hepatitis B virus (HBV) is a partially double stranded DNA virus with genetic diversity represented by eight genotypes (A to H). Natural course and response to treatment could be affected by HBV genotypes. HBV shows high rates of turn over in the absence of proof-reading ability. As a result, large amounts of quasi-species are produced naturally or antiviral-associated. HBV consists of four open reading frames, namely preS/S gene, precore/core gene, polymerase gene, and X gene. Mutations on preS gene can result in undetectable HBsAg even in case that HBV is replicating. Surface gene mutation leads to decreased binding affinity to anti-HBs, which is associated with a vaccine escape mutant. Precore mutation abolishes HBeAg whereas mutations on basal core promoter gene down-regulate the HBeAg production. Mutations on basal core promoter are associated with increased HBV replication and high incidence of progressive liver diseases such as liver cirrhosis and hepatocellular carcinoma. Mutations on polymerase genes are often induced by antiviral therapy. Emergence of antiviral-resistant mutation is the major cause of treatment failure. Furthermore, existence of prior antiviral-resistant mutations limits the options of subsequent antiviral agents. Therefore, judicious use of antivirals and selection of the most potent drug with the lowest resistance rate are of the utmost importance for the prevention of antiviral-associated mutants. Detailed knowledge and understanding of HBV genetic diversity and mutant would be critical to establish strategies for the diagnosis and management of HBV infection. (Korean J Hepatol 2008;14:446-464)
