Amino acid sequence analysis of the tryptic digest from the 89- and 80-kilodalton subunits of TFIIH revealed that these polypeptides corresponded to the XPB (ERCC-3) and XPD (ERCC-2) gene products, respectively, which are two presumed helicases implicated in the human DNA excision repair disorders XP and Cockayne syndrome (CS). This evidence concerns the gene GTF2H5 and Cockayne syndrome.