Unrooted neighbor-joining phylogeny based on amino-terminal portion of DNA Pol protein sequences as discussed in the text. Labels at branch tips represent taxa as presented in Fig. 1. Numbers at branch nodes indicate percent bootstrap support for that node based on 500 replications. (V. R. DeFilippis and L. P. Villarreal, J. Virol. 74:7079-7084, 2000.)