ERCC4 and xeroderma pigmentosum: In humans, defects in NER are causally linked to mutagenesis and cancer initiation as in the cancer-prone syndrome xeroderma pigmentosum (XP, complementation groups XP-A to XP-G)8 or to developmental and neuronal abnormalities as seen in a heterogeneous group of progeroid syndromes, including the Cockayne syndrome (CS; affected genes: Csa, Csa), Trichothiodystrophy (TTD; affected genes: Xpb, Xpd) or the XPF-ERCC1 syndrome (XFE; affected genes: Ercc1, Xpf)9–11.