Involved in DNA mismatch repair (MMR), correcting insertion-deletion loops (IDLs) resulting from DNA replication, DNA damage or from recombination events between non-identical sequences during meiosis. Component of the MutLbeta heterodimer, which probably forms a ternary complex with the MutSbeta heterodimer that initially recognizes the DNA mismatches. This complex is thought to be responsible for directing the downsteam MMR events, including strand discrimination, excision, and resynthesis. Plays a major role in promoting meiotic crossing-over and is involved in maintaining the genetic stability of simple sequence repeats by correction of frameshift intermediates. Heterodimer of MLH1 and PMS1, called MutLalpha, which is the major MMR MutL activity correcting base-base mismatches as well as IDLs. The heterodimer binds double strand DNA independently of a mismatch with positive cooperativity and has more than one DNA binding site. Heterodimer of MLH1 and MLH3, called MutLbeta, which is involved in correction of a specific subset of IDLs when associated with MutSbeta (By similarity). Ubiquitous. Expressed during prophase I of meiosis: detected from pachytene to diakinesis stages. Reduced fertility and mitotic defects: 72 per cent reduction in homologous somatic recombination. Belongs to the DNA mismatch repair MutL/HexB family.