Tfb5 interacts with Tfb2 and facilitates nucleotide excision repair in yeast（Nucleic Acids Research, 2007, Vol. 35, No. 3 861–871） ABSTRACTTFIIH is indispensable for nucleotide excision repair (NER) and RNA polymerase II transcription. Its tenth subunit was recently discovered in yeast as Tfb5. Unlike other TFIIH subunits, Tfb5 is note ssential for cell survival. We have analyzed ther ole of Tfb5 in NER. NER was deficient in the tfb5 deletion mutant cell extracts, and was specifically complemented by purified Tfb5 protein. In contrast to the extreme ultraviolet (UV) sensitivity of rad14 mutant cells that lack any NER activity, tfb5 deletionm utant cells were moderately sensitive to UVr adiation, resembling that of the tfb1-101 mutantc ells in which TFIIH activity is compromised butn ot eliminated. Thus, Tfb5 protein directly participates in NER and is an accessory NER protein that stimulates the repair to the proficient level. Lacking a DNA binding activity, Tfb5 was found toi nteract with the core TFIIH subunit Tfb2, but not with other NER proteins. The Tfb5–Tfb2 interaction was correlated with the cellular NER function ofT fb5, supporting the functional importance of this interaction. Our results led to a model in whichT b acts as an architectural stabilizer conferring tructural rigidity to the core TFIIH such that the complex is maintained in its functional architecture.