During the replication cycle, new viral proteins are produced and combined to produce new virions. Some of the hepatitis B virus proteins are produced in excess and exported out of the cell to form circulating viral proteins that lack any viral nucleic acid. The hepatitis B virus surface protein is produced in vast excess and the released proteins form circulating particles whose function is not clear. These ‘empty’ viral particles, which contain only the hepatitis B virus surface proteins, are easily detected by standard laboratory assays and they form the basis of the hepatitis B virus surface antigen (HBsAg) diagnostic assay. Hence the assay that is widely used to detect hepatitis B virus infection actually identifies ‘empty’ viral shells and not the infectious virus.