ERCC1 mediates DNA– and protein–protein interactions. XPF provides the endonuclease active site and is involved in DNA binding and additional protein–protein interactions. The ERCC4/XPF protein consists of two conserved domains separated by a less conserved region in the middle. The N-terminal region has homology to several conserved domains of DNA helicases belonging to superfamily II, although XPF is not a DNA helicase. The C-terminal region of XPF includes the active site residues for nuclease activity. Most of the ERCC1 protein is related at the sequence level to the C-terminus of the XPF protein, but residues in the nuclease domain are not present. A DNA binding “helix-hairpin-helix” domain at the C-terminus of each protein. By primary sequence and protein structural similarity, the ERCC1-XPF nuclease is a member of a broader family of structure specific DNA nucleases comprising two subunits. Such nucleases include, for example, the MUS81-EME1 nuclease. Structure-specific nuclease The ERCC1–XPF complex is a structure-specific endonuclease. ERCC1-XPF does not cut DNA that is exclusively single-stranded or double-stranded, but it cleaves the DNA phosphodiester backbone specifically at junctions between double-stranded and single-stranded DNA. It introduces a cut in double-stranded DNA on the 5′ side of such a junction, about two nucleotides away. This structure-specificity was initially demonstrated for RAD10-RAD1, the yeast orthologs of ERCC1 and XPF. The hydrophobic helix–hairpin–helix motifs in the C-terminal regions of ERCC1 and XPF interact to promote dimerization of the two proteins. There is no catalytic activity in the absence of dimerization. Indeed, although the catalytic domain is within XPF and ERCC1 is catalytically inactive, ERCC1 is indispensable for activity of the complex. Several models have been proposed for binding of ERCC1–XPF to DNA, based on partial structures of relevant protein fragments at atomic resolution. DNA binding mediated by the helix-hairpin-helix domains of ERCC1 and XPF domains positions the heterodimer at the junction between double-stranded and single-stranded DNA. Nucleotide excision repair During nucleotide excision repair, several protein complexes cooperate to recognize damaged DNA and locally separate the DNA helix for a short distance on either side of the site of a DNA damage. The ERCC1–XPF nuclease incises the damaged DNA strand on the 5′ side of the lesion. During NER, the ERCC1 protein interacts with the XPA protein to coordinate DNA and protein binding. DNA double-strand break repair Mammalian cells with mutant ERCC1–XPF are moderately more sensitive than normal cells to agents (such as ionizing radiation) that cause