ERVW-1 and infection: Membrane fusion is an essential step of infection for enveloped viruses such as HIV-1, and requires extensive conformational rearrangements of the Env prefusion conformation (Julien et al., 2013; Lyumkis et al., 2013; Pancera et al., 2014) into the final inactive post-fusion conformation (Harrison, 2008; Weissenhorn et al., 2007).