ERCC2 and xeroderma pigmentosum: The XPD gene, encoding a helicase subunit of the transcription/NER-associated transcription factor II H complex, is unique amongst NER genes in that different point mutations are associated with cancer (XP), progeria (TTD), or a combination (XP combined with Cockayne syndrome [XPCS] or trichothiodystrophy [XPTTD]) [8,14,15].