Xeroderma pigmentosum type B (XPB) and type D (XPD) DNA helicases are involved in transcription-coupled DNA repair (Drapkin et al., 1994; Coin et al., 2007), and mutations in XPB and XPD helicases are found in patients with Cockayne syndrome (CS), trichothiodystrophy (TTD), and xeroderma pigmentosum (XP) (Lehmann A. R, 2001; Kraemer et al., 2007). The gene discussed is ERCC2; the disease is Cowden syndrome 1.