Departament de Genètica i Servei de Bioinformàtica, Universitat de València, Spain.
We report a phylogenetic study of viroids, some plant satellite RNAs, and the viroidlike domain of human hepatitis delta virus RNA. Our results support a monophyletic origin of these RNAs and are consistent with the hypothesis that they may be "living fossils" of a precellular RNA world. Moreover, the viroidlike domain of human hepatitis delta virus RNA appears closely related to the viroidlike satellite RNAs of plants, with which it shares some structural and functional properties. On the basis of our phylogenetic analysis, we propose a taxonomic classification of these RNAs.