Hepatitis B virus is an hepadnavirus and it has a circular genome composed of partially double-stranded DNA. The viruses replicate through an RNA intermediate form by reverse transcription, and in this respect they are similar to retroviruses. During the 'window' in which the host remains infected but is successfully clearing the virus, IgM antibodies to the hepatitis B core antigen (anti-HBc IgM) may be the only serological evidence of disease. Alternative Names: Hepatitis B core protein, HBcAg antigen