ERVW-1 and infection: Since the secreted HBsAg is known to be in great excess over virions during natural HBV infection [27,31], the higher levels of HBsAg produced from pSVHBV1.5 helped to ensure that the complementation experiment mimicked better the natural infection in terms of HBsAg expression and to avoid the potential situation where the expression of the envelope proteins might become limiting for virion secretion.