XPB (Xeroderma pigmentosum group B complementing protein; also ERCC3 and TFIIH 89kD subunit) is an 86kD, nuclear member of the RAD25/XPB subfamily, helicase family of proteins. It is expressed in multiple cell types, including neurons, fibroblasts and keratinocytes. XPB is part of the 10 subunit TFIIH complex. As such, it appears to participate in both nucleotide excision repair (NER) and transcription initiation from RNA polymerase II promoters. During NER, XPB utilizes an intrinsic ATPase activity to resolve nucleotide excision and repair. Human XPB is 782aa in length. It contains an NLS (aa6-18), multiple acidic-rich regions, an ATPase region (aa 327-488) and one helicase domain (aa542-702). Over (aa1-375), human XPB shares 93% aa identity with mouse XPB.