[viruses]
   Order Ortervirales
     Family Retroviridae
       Subfamily Orthoretrovirinae
         Genus Alpharetrovirus
           1 Avian carcinoma Mill Hill virus 2
           2 Avian leukosis virusᵀ
           3 Avian myeloblastosis virus
           4 Avian myelocytomatosis virus 29
           5 Avian sarcoma virus CT10
           6 Fujinami sarcoma virus
           7 Rous sarcoma virus
           8 UR2 sarcoma virus
           9 Y73 sarcoma virus