<p>Xeroderma pigmentosum (XP) [<cite idref="PUB00005433"/>] is a human autosomal recessive disease, characterised by a high incidence of sunlight-induced skin cancer. People's skin cells with this condition are hypersensitive to ultraviolet light, due to defects in the incision step of DNA excision repair. There are a minimum of seven genetic complementation groups involved in this pathway: XP-A to XP-G. XP-G is one of the most rare and phenotypically heterogeneous of XP, showing anything from slight to extreme dysfunction in DNA excision repair [<cite idref="PUB00004415"/>, <cite idref="PUB00002851"/>]. XP-G can be corrected by a 133 Kd nuclear protein, XPGC [<cite idref="PUB00005433"/>]. XPGC is an acidic protein that confers normal UV resistance in expressing cells [<cite idref="PUB00002851"/>]. It is a magnesium-dependent, single-strand DNA endonuclease that makes structure-specific endonucleolytic incisions in a DNA substrate containing a duplex region and single-stranded arms [<cite idref="PUB00002851"/>, <cite idref="PUB00004189"/>]. XPGC cleaves one strand of the duplex at the border with the single-stranded region [<cite idref="PUB00004189"/>].</p><p>XPG belongs to a family of proteins that includes RAD2 from <taxon tax_id="4932">Saccharomyces cerevisiae</taxon> (Baker's yeast) and rad13 from <taxon tax_id="4896">Schizosaccharomyces pombe</taxon> (Fission yeast), which are single-stranded DNA endonucleases [<cite idref="PUB00004189"/>, <cite idref="PUB00004160"/>]; mouse and human FEN-1, a structure-specific endonuclease; RAD2 from fission yeast and RAD27 from budding yeast; fission yeast exo1, a 5'-3' double-stranded DNA exonuclease that may act in a pathway that corrects mismatched base pairs; yeast DHS1, and yeast DIN7. Sequence alignment of this family of proteins reveals that similarities are largely confined to two regions. The first is located at the N-terminal extremity (N-region) and corresponds to the first 95 to 105 amino acids. The second region is internal (I-region) and found towards the C terminus; it spans about 140 residues and contains a highly conserved core of 27 amino acids that includes a conserved pentapeptide (E-A-[DE]-A-[QS]). It is possible that the conserved acidic residues are involved in the catalytic mechanism of DNA excision repair in XPG. The amino acids linking the N- and I-regions are not conserved.</p><p>The XP group D gene product (XPD) is a helicase that is required for nucleotide excision repair, and is also one of the components of basal transcription factor TFIIH [<cite idref="PUB00002008"/>, <cite idref="PUB00003894"/>]. DNA repair defects in the XPD group are associated with the clinical features of XP and trichothiodystrophy (TTD), which is characterised by sulphur-deficient brittle hair and a variety of other associated abnormalities, but no skin cancer [<cite idref="PUB00003894"/>].</p><p>XPD belongs to a family of ATP-dependent helicases that are characterised by a 'D-E-A-H' motif [<cite idref="PUB00004395"/>]. 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.</p> Xeroderma pigmentosum group D protein