The present invention pertains to a nucleic acid sequence encoding a human TFIIH 8 kDa subunit and related sequences. The nucleic acids may be used in methods for producing a TFIIH subunit, as well as in methods for diagnosing or treating transcription and NER deficiencies, in particular in some forms of trichothiodystrophy (TTD). The hTFB5/TTDA gene and encoded protein may be used for therapy or genetherapy products, aimed at treating congenital NER disorders and may also be used in methods of diagnosis of disorders in basal transcription, NER and TCR activity in mammals, using molecular probes or antibodies specific for TTDA.