Crossover formations are achieved by the results of homologous recombination. Homologous recombination starts from the induction of DNA double-strand breaks (DSBs). Resection and single-strand invasion works to synthesize new DNA with developing D-loops. The synthesized singe-end can be annealed with second-end to make a non crossover product. The process is called synthesis-dependent strand annealing, SDSA (A). Developed-D-loops can be annealed with the second-end as well. At this point, D-loop/nicked Holliday junctions are cleaved by structure-specific endonucleases (SSEs), such as MUS-81-EME-1 to form crossover products (B). The ligation process occurs after second-end capture forms double Holliday Junctions. The same directional branch migration and decatenation (dissolution) results in non-crossover formation (C). Theoretically, there is a 50-50 chance to make crossover and non-crossover products via double Holliday junction resolution by SSEs such as HIM-18/SLX4-SLX-1, XPF-1-ERCC-1 (D and E). Interestingly, crossover specific resolution is reinforced at the site of designation for crossover formation by pro crossover factors. We are investigating the mechanisms of the crossover/non-crossover decision.