<p>A number of eukaryotic and prokaryotic proteins have been characterised [<cite idref="PUB00003826"/>, <cite idref="PUB00004037"/>, <cite idref="PUB00004089"/>] on the basis of their structural similarity. They all seem to be involved in ATP-dependent, nucleic-acid unwinding. There are two subfamilies of such proteins, the D-E-A-D-box and D-E-A-H-box families.Proteins that belong to the subfamily which have His instead of the second Asp are said to be 'D-E-A-H-box' proteins [<cite idref="PUB00004089"/>, <cite idref="PUB00004395"/>, <cite idref="PUB00003151"/>]. </p> <p>Proteins currently known to belong to this subfamily includeyeast PRP2, PRP16, PRP22 and PRP43, involved in various ATP-requiring steps of the pre-mRNA splicing process; fission yeast prh1, which my be involved in pre-mRNA splicing; Drosophila male-less (mle) protein required in males for dosage compensation of X chromosome linked genes; yeast RAD3, a DNA helicase involved in excision repair of DNA damaged by UV light, bulky adducts or cross-linking agents; fission yeast rad15 (rhp3) and mammalian DNA excision repair protein XPD (ERCC-2);yeast CHL1 (or CTF1), which is important for chromosome transmission and normal cell cycle progression in G(2)/M; yeast TPS1, <taxon tax_id="6239">Caenorhabditis elegans</taxon> hypothetical proteins C06E1.10 and K03H1.2; Poxviruses' early transcription factor 70kDa subunit which acts with RNA polymerase to initiate transcription from early gene promoters; <taxon tax_id="10245">Vaccinia virus</taxon> putative helicase I8; and <taxon tax_id="562">Escherichia coli</taxon> putative RNA helicase hrpA.All these proteins share a number of conserved sequence motifs. Some of them are specific to this family while others are shared by other ATP-binding proteins or by proteins belonging to the helicases `superfamily' [<cite idref="PUB00004021"/>].</p>