Two primary models for how homologous recombination repairs double-strand breaks in DNA are the double-strand break repair (DSBR) pathway (sometimes called the "double Holliday junction model") and the synthesis-dependent strand annealing (SDSA) pathway. The two pathways are similar in their first several steps. After a double-strand break occurs, the MRX complex (MRN complex in humans) binds to DNA on either side of the break. Next a resection takes place, in which DNA around the 5' ends of the break is cut back. This happens in two distinct steps: first the MRX complex recruits the Sae2 protein, and these two proteins trim back the 5' ends on either side of the break to create short 3' overhangs of single-strand DNA; in the second step, 5'→3' resection is continued by the Sgs1 helicase and the Exo1 and Dna2 nucleases. As a helicase, Sgs1 "unzips" the double-strand DNA, while the nuclease activity of Exo1 and Dna2 allows them to cut the single-stranded DNA produced by Sgs1.