The human hepatitis B virus (HBV) elicits acute and chronic liver disease in humans and has been linked to hepatocellular carcinoma. The virus is transmitted by perinatal exposure, sexual contact, exposure to blood products and organ/tissue transplantation. HBV is a member of the hepadnavirous family and the viral genome within the HBV particle is a circular, partially double-stranded, relaxed-circular DNA molecule. The infectious particles are called Dane particles and consist of an icosahedral nucleocapsid surrounded by a lipoprotein envelope. The promoter, enhancer, poly- A addition signal and glucocorticoid-responsive element (GRE) are signal sexuences which are involved in HBV gene expression. The virus replicates via an RNA pre-genome using covalently closed, circular, double stranded DNA as a transcriptional template. Multiple mutant variants have been identified and characterized at the molecular level with certain clinical syndromes.