Xeroderma pigmentosum patients belonging to the XPF complementation group have reduced cellular levels of both XPF and ERCC1 and, conversely, ERCC1-deficient cells show low levels of XPF, implying that formation of the heterodimeric complex stabilises both proteins in vivo [17, 18]. This evidence concerns the gene ERCC4 and xeroderma pigmentosum.