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 ERCC2 and Cowden syndrome 1.