ERVW-1 and infection: To ensure that only a single-cycle of infection occurred in these experiments, we constructed an envelope-deficient Tas-2A-tagRFP reporter virus (PFVΔEnv-2A-RFP) in which both Tas and tagRFP were expressed from the FV internal promoter (Fig 4A) and generated virions by co-transfection of the envelope-deficient proviral plasmid with a PFV-Env expression plasmid.