Involved in meiotic recombination. Facilitate crossovers between homologs during meiosis. Heterooligomer of MSH4 and MSH5. Two distinct classes of crossovers have been demonstrated in budding yeast. Class I is MSH4/MSH5 dependent and exhibits crossover interference. Class II is MUS81/MMS4 dependent and exhibits no interference. Belongs to the DNA mismatch repair MutS family.