ERVW-1 and infection: Secondly, we consider that Lineage B CrERVs could increase in the genome by infection if the cocirculating Lineage A group provided a functional envelope protein, a process called complementation (Mager and Freeman 1995; Belshaw, Katzourakis, et al. 2005) This latter mechanism requires that a member of each CrERV lineage be transcriptionally active at the same time in the same cell, and that intact proteins from the “helper” genome be used to assemble a particle with a functional envelope for reinfection.