CDK7 and xeroderma pigmentosum group D: TFIIH is composed of two sub-complexes: the 7-subunit core complex comprised of xeroderma pigmentosum group B (XPB), xeroderma pigmentosum group D (XPD), p62, p52, p44, p34 and trichothiodystrophy group A (TTDA), and the associated trimeric CDK-activating kinase (CAK) complex involving CDK7, MAT1 and cyclin H.