ERVW-1 and infection: Second, in order to allow selective targeting of the initial infection to the group of neurons of interest, the viral particles are coated with the envelope protein of a different virus (the avian-endemic retrovirus “ASLV-A”; the envelope protein of which is called “EnvA”), rendering the virus incapable of infecting mammalian neurons without assistance (Wickersham et al., 2007b).