The outcome of HTLV-1 and HIV-1 pseudotyping, particularly the production of HIV-1 virions with HTLV-1 Env, can broaden the spectrum of HIV-1 target cells; in this case, only a single round of infection can be completed, unless there is a concomitant superinfection with wt HTLV-1, able to produce new pseudotyped HIV-1 virions. This evidence concerns the gene ERVW-1 and infection.