A remarkable structure-specific DNA-nuclease family in eukaryotes is represented by the heterodimeric XPF–ERCC1 complex in mammalian cells 1 Sijbers A.M et al. Cell. 1996; 86: 811-822 Abstract Full Text Full Text PDF PubMed Scopus (441) Google Scholar and by the Rad1p–Rad10p complex in Saccharomyces cerevisiae2 Bardwell A.J Bardwell L Tomkinson A.E Friedberg E.C Science. 1994; 265: 2082-2085 Crossref PubMed Scopus (222) Google Scholar . These enzymes specifically cleave DNA near junctions between single-stranded and duplex DNA in cases where the single strand (moving away from the junction) has a 5′–3′ polarity. The ability of these enzymes to cut such bubble, flap and Y structures is necessary for many important DNA transactions in the cell. For example, nucleotide-excision repair in eukaryotes uses the nuclease to cleave a damaged DNA strand on the 5′ side of a lesion 1 Sijbers A.M et al. Cell. 1996; 86: 811-822 Abstract Full Text Full Text PDF PubMed Scopus (441) Google Scholar , 2 Bardwell A.J Bardwell L Tomkinson A.E Friedberg E.C Science. 1994; 265: 2082-2085 Crossref PubMed Scopus (222) Google Scholar , 3 Mu D Hsu D.S Sancar A J. Biol. Chem. 1996; 271: 8285-8294 Abstract Full Text Full Text PDF PubMed Scopus (339) Google Scholar . A major form of repair of double-stranded breaks involves the use of Rad1p–Rad10p, which form a complex with the Msh2 and Msh3 mismatch repair proteins, to resolve recombination intermediates 4 Paques F Haber J.E Mol. Cell. Biol. 1997; 17: 6765-6771 Crossref PubMed Scopus (178) Google Scholar . Further, XPF and ERCC1 homologs are involved in meiosis in Drosophila melanogaster5 Sekelsky J.J McKim K.S Chin G.M Hawley R.S Genetics. 1995; 141: 619-627 PubMed Google Scholar and in mating-type switching in Schizosaccharomyces pombe6 Carr A et al. Mol. Cell. Biol. 1994; 14: 2029-2040 Crossref PubMed Scopus (75) Google Scholar .