Adding even more to the complexity, mutations in NER factors such as XPD that are employed by both GG- and TC-NER can lead to XP (typically with neurodegenerative components), rare cases of XP combined with CS, as well as trichothiodystrophy (TTD), where patients suffer from growth and mental retardation and premature aging, in addition to displaying typical brittle hair and nails that have been directly linked to transcription defects resulting from XPD or TTDA mutations [10]. The gene discussed is ERCC2; the disease is xeroderma pigmentosum.