<p> This domain of unknown function is found in the Xeroderma pigmentosum group D (XPD) proteins which belong to a family of ATP-dependent helicases characterised by a 'D-E-A-H' motif. This resembles the 'D-E-A-D-box' of other known helicases, which represents a special version of the B motif of ATP-binding proteins. In XPD, His replaces the second Asp. The DEAD box helicases are involved in various aspects of RNA metabolism, including nuclear transcription, pre-mRNA splicing, ribosome biogenesis, nucleocytoplasmic transport, translation, RNA decay and organellar gene expression. </p>