Fig. 1.

Envelope-based phylogenetic tree with positions of syncytin-A and -B. The tree was determined by the neighbor-joining method by using TM envelope sequences (see ref. 17), essentially from murine ERVs (in italics) and human HERVs. The horizontal branch length and scale indicate the percentage of amino acid substitutions from the node, and the arrow on the left points to the node of the CKS17-positive sequence group. Percent bootstrap values obtained from 1,000 replicates are indicated. Accession numbers for each representative element together with the sequence alignment used to generate the tree are given in Fig. 8, which is published as supporting information on the PNAS web site.