ERCC1 and xeroderma pigmentosum: In mammalian cells, null mutants of the ERCC1 or XPF genes are lethal and weaker mutations can result in xeroderma pigmentosum (XP), trichothiodystrophy (TTD), and Cockayne syndrome (CS), genetic disorders that are typical of mutations in genes required for NER (Gregg et al., 2011).