ATP-dependent 3'-5' DNA helicase/translocase (PubMed:17466626, PubMed:27193682, PubMed:33902107, PubMed:8465201, PubMed:8663148). Binds dsDNA rather than ssDNA, unzipping it in a translocase rather than classical helicase activity (PubMed:27193682, PubMed:33902107). Component of the general transcription and DNA repair factor IIH (TFIIH) core complex (PubMed:10024882, PubMed:17466626, PubMed:8157004, PubMed:8465201). When complexed to CDK-activating kinase (CAK), involved in RNA transcription by RNA polymerase II. The ATPase activity of XPB/ERCC3, but not its helicase activity, is required for DNA opening; it may wrap around the damaged DNA wedging it open, causing localized melting that allows XPD/ERCC2 helicase to anchor (PubMed:10024882, PubMed:17466626). In transcription, TFIIH has an essential role in transcription initiation (PubMed:30894545, PubMed:8157004). When the pre-initiation complex (PIC) has been established, TFIIH is required for promoter opening and promoter escape (PubMed:8157004). The ATP-dependent helicase activity of XPB/ERCC3 is required for promoter opening and promoter escape (PubMed:10024882). In transcription pre-initiation complexes induces and propagates a DNA twist to open DNA (PubMed:27193682, PubMed:33902107). Also involved in transcription-coupled nucleotide excision repair (NER) of damaged DNA (PubMed:17466626, PubMed:2111438, PubMed:8157004). 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. The structure of the TFIIH transcription complex differs from the NER-TFIIH complex; large movements by XPD/ERCC2 and XPB/ERCC3 are stabilized by XPA (PubMed:31253769, PubMed:33902107). XPA retains XPB/ERCC3 at the 5' end of a DNA bubble (mimicking DNA damage) (PubMed:31253769). {ECO:0000269|PubMed:10024882, ECO:0000269|PubMed:17466626, ECO:0000269|PubMed:30894545, ECO:0000269|PubMed:31253769, ECO:0000269|PubMed:33902107, ECO:0000269|PubMed:7724549, ECO:0000269|PubMed:8157004, ECO:0000269|PubMed:8663148, ECO:0000305|PubMed:8465201}. This is the function of ERCC3 (ERCC excision repair 3, TFIIH core complex helicase subunit, ENSG00000163161).