Mutations in the XPD helicase gene result in XP and TTD phenotypes, preventing interaction between XPD and the p44 subunit of TFIIH.

In most cases, xeroderma pigmentosum group D (XP-D) and trichothiodystrophy (TTD) patients carry mutations in the carboxy-terminal domain of the evolutionarily conserved helicase XPD, which is one of the subunits of the transcription/repair factor TFIIH (refs 1,2). In this study, we demonstrate that XPD interacts specifically with p44, another subunit of TFIIH, and that this interaction results in the stimulation of 5'-->3' helicase activity. Mutations in the XPD C-terminal domain, as found in most patients, prevent the interaction with p44, thus explaining the decrease in XPD helicase activity and the nucleotide excision repair (NER) defect.