The Viruses
   The DNA and RNA Reverse Transcribing Viruses
     Family Retroviridae
       Subfamily Orthoretrovirinae
         Genus Betaretrovirus
           1 Jaagsiekte sheep retrovirus
           2 Langur virus
           3 Mason-Pfizer monkey virus
           4 Mouse mammary tumor virusᵀ
           5 Squirrel monkey retrovirus