[viruses]
   Order Ortervirales
     Family Retroviridae
       Subfamily Orthoretrovirinae
         1 Genus Alpharetrovirus
         2 Genus Betaretrovirus
         3 Genus Deltaretrovirus
         4 Genus Epsilonretrovirus
         5 Genus Gammaretrovirus
         6 Genus Lentivirus