Phylogenetic comparison of the envelope (Env) protein sequence of betaretroviruses and gammaretroviruses. Bootstrap values <70% are not shown, and branch lengths are drawn to a scale of amino acid substitutions per site. Bootstrap values are denoted as ** >90%; * >70% and <90%. βERV proteins of P. vampyrus and P. alecto are highlighted in red text. βERVs of M. lucifugus and R. ferrumequinum are highlighted in blue text. Gammaretroviruses are highlighted in teal text.