Required during the pachytene stage of meiotic prophase for the formation of crossovers between homologous chromosomes. Together with msh-5 and zhp-3 plays a role in the activation of DNA damage-dependent apoptosis at the DNA damage checkpoint in pachytene cells (PubMed:23832114). Not needed for pairing or synapsis. May promote crossing over by interfering with Holliday junction branch migration. Has no apparent role in DNA mismatch repair. Heterooligomer of him-14 and msh-5. Expressed maternally. Worms show maternal effect lethality producing 95% inviable progeny from hermaphrodites. A high proportion of those that survive are male. Mutants display severely reduced crossing over, resulting in lack of chiasmata between homologous chromosomes and consequent missegregation. Belongs to the DNA mismatch repair MutS family.