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. The hepatitis B surface antigen (HBsAg) is most frequently used to screen for the presence of this infection. It is the first detectable viral antigen to appear during infection. Alternative Names: Hepatitis B surface Ag protein, Hepatitis B Surface Ag Subtype ayw protein, Hepatitis B Surface Antigen protein