ERVW-1 and HIV-1 infection: This procedure relies on acute HIV-1 infection with a fully competent R5-tropic reporter virus coding for the murine surface molecule HSA (NL4.3 HSA-IRES-Nef Bal-env; called NL/Bal-HSA hereafter)13, 14 coupled with an immunomagnetic sorting method (Fig. 1A).