TFIIH is a multiprotein complex that has a central role in the RNA pol II mediated transcription, in DNA repair and in the control of the cell cycle. TFIIH consists of nine subunits: cyclin-dependent kinase 7 (Cdk7), cyclin H and MAT1 (forming the Cdk-activating-kinase or CAK complex), the two helicases Xpb/Hay and Xpd, and p34, p44, p52 and p62. As the kinase subunit of TFIIH, Cdk7 participates in basal transcription by phosphorylating the carboxy-terminal domain of the largest subunit of RNA polymerase II. Mutations in some components of TFIIH are associated with three hereditary human syndromes: xeroderma pigmentosum (XP), Cockayne syndrome (CS) and trichothiodystrophy (TTD). The p62 protein (TFIIH1) is a structural component of the TFIIH core and no syndromes have been identified up to date by mutations in this human gene.