While the F-MLV studies do not definitively prove that Rfv3 is mA3, the fact that mA3−/− mice are more susceptible to infection by at least two murine retroviruses, MMTV and MLV, and that the loss of this gene in vivo leads to increased pathogenesis by these viruses, provides strong support that this host-encoded restriction factor does function against a natural pathogen. This evidence concerns the gene PNMA3 and infection.