In budding yeast "Saccharomyces cerevisiae", Holliday junctions can be resolved by four different pathways that account for essentially all Holliday junction resolution in vivo. The pathway that produces the majority of crossovers in "S. cerevisiae" budding yeast, and possibly in mammals, involves proteins EXO1, MLH1-MLH3 heterodimer (called MutL gamma) and SGS1 (ortholog of Bloom syndrome helicase). The MLH1-MLH3 heterodimer binds preferentially to Holliday junctions. It is an endonuclease that makes single-strand breaks in supercoiled double-stranded DNA. The MLH1-MLH3 heterodimer promotes the formation of crossover recombinants. While the other three pathways, involving proteins MUS81-MMS4, SLX1 and YEN1, respectively, can promote Holliday junction resolution in vivo, absence of all three nucleases has only a modest impact on formation of crossover products.