The hepatitis B core antigen (HBcAg) is a protein expressed by the hepatitis B virus (HBV). The 262-aa protein self assembles to form the virion icosahedral capsid, with 180 to 240 protein copies per capsid. Comprising the inner layer nucleocapsid, the HBV core antigen is used as a marker for the presence of viable, replicating virions. A variety of monoclonal and polyclonal HBcAg antibodies are available from various suppliers.