Phylogenetic tree comparing 662 bases of pol DNA from human (HTLV) and simian (STLV), PTLV strains and five BLV strains. Bootstrap values for all except the most terminal branches are greater than 90%. The bootstrap value for the branch that contains BLV Arg 41 and BLV Arg 38 is 100%. The bar at the lower left indicates the length of a 10% distance between sequences. As can be seen, the PTLV segregate into four distinct species, while the BLV strains constitute a single species.