In the plant "Arabidopsis thaliana", homologs of the Sgs1(BLM) helicase act as major barriers to meiotic crossover formation. These helicases are thought to displace the invading strand allowing its annealing with the other 3’overhang end of the double-strand break, leading to non-crossover recombinant formation by a process called synthesis-dependent strand annealing (SDSA) (see Wikipedia article “Genetic recombination”). It is estimated that only about 5% of double-strand breaks are repaired by crossover recombination. Sequela-Arnaud et al. suggested that crossover numbers are restricted because of the long-term costs of crossover recombination, that is, the breaking up of favorable genetic combinations of alleles built up by past natural selection.