Intriguingly, to date, mutations affecting only the XPB, XPD and p8 subunits have been identified in human patients afflicted with XP, CS or TTD [10–13], suggesting that mutations in other TFIIH subunits in humans may be more deleterious, or are indistinguishable from the wild-type phenotype. The gene discussed is ERCC2; the disease is xeroderma pigmentosum.