The hepatitis B surface antigen (HBsAg) is an envelope protein found on the surface of the HBV virion. It mediates receptor binding and recognition to liver host cells, and probably is involved with membrane fusion as well. There are two known forms: large HBsAg (330-aa) and small HBsAg (167-aa). HBsAg is a common serum antigen to detect for HBV infection, as well as for monitoring virion levels in response to treatment. Recombinant versions of antigen have also been used to produce hepatitis vaccines.