Finally, to permit longitudinal monitoring of progressing infection, we generated replication-competent MLV reporter viruses by introducing a shortened internal ribosome entry site (IRES), 6ATRi, to drive Nluc expression downstream of viral Env (MLV 6ATRi-Nluc) (Ventura et al., 2019; Alberti et al., 2015; Logg et al., 2001). This evidence concerns the gene ERVW-1 and infection.