Hepatitis B virus (HBV) is a member of the Hepadnavirus family. The little virion (diameter of 42nm) consists of an outer lipid envelope and an icosahedral nucleocapsid core composed of protein. The nucleocapsid encloses the viral DNA and a DNA polymerase that has reverse transcriptase activity. The outer envelope contains embedded proteins which are involved in viral binding of susceptible cells. Also exist pleomorphic forms, as filamentous and spherical bodies lacking a core. These particles are not infectious and are composed of the lipid and protein that forms part of the surface of the virion, which is called the surface antigen (HBsAg), and is produced in excess during the life cycle of the virus. Although Hepatitis A virus, Hepatitis B virus, and Hepatitis C virus have similar names (because they all cause liver inflammation), these are distinctly different viruses both genetically and clinically.