In order for repair of the DNA double-strand breaks to occur through homologous recombination or single strand annealing, the 5' ends of the break must first be resected to produce 3' overhanging single stranded DNA (ssDNA) that can subsequently invade homologous duplex DNA (e.g. in the sister chromatid) (Thompson et al. 2001, Kolpashchikov et al. 2001). The MRE11A component of the MRN complex possesses endonuclease activity (Trujillo et al. 1998, Hopfner et al. 2002) that is activated by binding of RBBP8 (CtIP) and BRCA1, in the presence of Mn2+ or Mg2+ ions (Sartori et al. 2007, Yun and Hiom 2009).