In this week's PNAS Early Edition, Eugene Gladyshev and Irina Arkhipova at the Marine Biological Laboratory in Woods Hole, Mass., present evidence that suggests the class of class of reverse transcriptase-related cellular genes collectively called rvts "are not components of retrotransposons or viruses, but [are] single-copy genes with a characteristic domain structure that may contain introns in evolutionarily conserved positions, occur in syntenic regions, and evolve under purifying selection." Gladyshev and Arkhipova show that these genes "can be found in all major taxonomic gro