represents a unique lineage of virus-like retroelements.

Eukaryotic reverse transcriptase genes are mostly incorporated into viruses or transposons. Among the six reported families of reverse-transcribing viruses, three families (/, / and /) have proliferated mostly as transposons, collectively known as LTR retrotransposons. was reported as a unique lineage of the / family. While most LTR retrotransposons generate 4 to 6 bp target site duplications (TSDs) upon integration and contain 5'-TG. CA-3' termini, generates 3 bp TSDs and contains 5'-CG.CG-3' termini. Here, the distribution and diversity of were extensively investigated from the available genome sequences. In addition to the six animal phyla reported previously, was characterized for the first time in Hemichordata, Priapulida, Annelida, Phoronida and Brachiopoda. The unique terminal nucleotides of are very well conserved. The phylogenetic analysis with various combinations of conserved domains supported the independent position of from any established retroelement groups, most likely as the sister lineage of the group including /, , , Lokiretroviruses and LTR retrotransposons. is here proposed as a new superfamily of LTR retrotransposons.