[viruses]
   Order Ortervirales
     Family Retroviridae
       Subfamily Orthoretrovirinae
         Genus Deltaretrovirus
           1 Bovine leukemia virusᵀ
           2 Primate T-lymphotropic virus 1
           3 Primate T-lymphotropic virus 2
           4 Primate T-lymphotropic virus 3