Models of Class I/Class II crossover pathways in mammals. A: D-loop formation can either be processed by the Class I pathway, whereby MutSɣ stabilization and MutLɣ resolution can occur. If processed by the Class II pathway, the intermediate will be cleaved by Mus81 to generate a crossover. Gray arrow: In the absence of Mus81, Class II intermediates are stabilized by MutSɣ and resolved by MutLɣ. B: Alternatively, MutSɣ stabilizes the recombination intermediate prior to Class I/Class II processing. Gray arrow: In the absence of Mus81, MutLɣ resolves the intermediate. Blm is not illustrated but it could have a disassembly function similar to that of orthologs in other model organisms.