The hepatitis B envelope antigen (HBeAg), is a HBV protein localized in between the virion capsid and its lipid envelope. The canonical protein has an amino acid length of 305 and a predicted mass of 34.8 kDa. There is one other noted isoform. The protein is believed to play an immunoprotective role that protects infected host cells from cytotoxic T-cells. HBeAg is secreted in the serum and is used for detecting HBV, as well as monitoring virus levels. Conjugated and unconjugated antibodies to HBeAg are available from a number of suppliers.