Reverse transcriptases occur in a variety of mobile elements, including retrotransposons, retroviruses, group II introns, bacterial msDNAs, hepadnaviruses and caulimoviruses [<cite idref="PUB00001193"/>]. The enzyme catalyses RNA-template- directed extension of the 3'-end of DNA strands one deoxynucleotide at a time; it cannot initiate a chain <i>de novo</i>, but requires an RNA or DNA primer, or DNA template.<p>The structure of the unliganded reverse transcriptase (RT) from the <taxon tax_id="11676">Human immunodeficiency virus 1</taxon> has been solved to 3.2A resolution [<cite idref="PUB00008071"/>]. The enzyme is a heterodimer, with domains termed "fingers," "thumb," "palm," and "connection" in both subunits, and a ribonuclease H domain in the larger subunit only [<cite idref="PUB00008071"/>]. Ligand binding has been shown to induce changes in the orientation of the thumb. A C-terminal domain common to a group of sequences from the <taxon tax_id="6239">Caenorhabditis elegans</taxon> sequencing project is revealed by sequence analysis to share a high level of similarity with the RT small subunit.</p> RNA-directed DNA polymerase, eukaryota