Component of the general transcription and DNA repair factor IIH (TFIIH) core complex, which is involved in general and transcription-coupled nucleotide excision repair (NER) of damaged DNA and in RNA transcription by RNA polymerase II (By similarity). In NER, TFIIH acts by opening DNA around the lesion to allow the excision of the damaged oligonucleotide and its replacement by a new DNA fragment (By similarity). In transcription, TFIIH has an essential role in transcription initiation (By similarity). When the pre-initiation complex (PIC) has been established, TFIIH is required for promoter opening and promoter escape (By similarity). Necessary for the stability of the TFIIH complex and for the presence of normal levels of TFIIH in the cell (PubMed:34824371). Required for efficient binding of TFIIH to damaged DNA (PubMed:34824371). Dispensable for normal development, but required when transcription is challenged (PubMed:34824371). Component of the 7-subunit TFIIH core complex. Localized to paired homologous chromosomes (bivalents) about 10 minutes after UV irradiation, but localization is lost after 35 minutes, probably as a result of successful DNA damage repair as part of the TFIIH complex. Auxin-induced degradation of the protein causes no obvious phenotype in germ cells and early embryos. Belongs to the TFB5 family.