We previously described the use of quantitative proteomics to study macromolecular complexes. Applying the method to analyze a yeast RNA polymerase II preinitiation complex, we identified a new 8-kDa protein, encoded by the uncharacterized open reading frame YDR079c-a, as a potential new component of the preinitiation complex. Here we show that YDR079c-a is a bona fide component of polymerase II preinitiation complexes and investigate its role in transcription. YDR079c-a is recruited to promoters both in vivo and in vitro and is required for efficient transcription in vitro and for normal induction of GAL genes. In addition, YDR079c-a is a core component of general transcription and DNA repair factor IIH and is required for efficient recruitment of TFIIH to a promoter. Yeast lacking YDR079c-a grow slowly, and, like strains carrying mutations in core TFIIH subunits, are sensitive to ultraviolet radiation. YDR079c-a is conserved throughout evolution, and mutations in the human ortholog account for a DNA repair-deficient form of the tricothiodystrophy disorder called TTD-A(2). The identification of a new, evolutionarily conserved, core TFIIH subunit is essential for our understanding of TFIIH function in transcription, DNA repair and human disease.