MLH3 (mutL homolog 3) is a DNA mismatch repair (MMR) protein that plays crucial roles in both somatic DNA repair and meiotic recombination. As part of the MMR system, MLH3 works alongside other MMR proteins including MLH1, PMS2, MSH2, and MSH6 to repair mismatched bases that arise during DNA replication 1. During meiosis, MLH3 forms the MutLγ nuclease complex with MLH1, which is essential for crossover formation and proper chromosome 14 2. This complex is regulated by MSH4-MSH5 (MutSγ), EXO1, RFC, and PCNA, and preferentially processes Holliday junctions through DNA nicking rather than canonical resolvase activity 2. MLH3 is critical for male fertility, as HSF5 regulates MLH3 transcription during meiotic progression, and disruption leads to spermatogenic arrest 3. Clinically, MLH3 deficiency is associated with colorectal cancer susceptibility, particularly in hereditary nonpolyposis colorectal cancer, though its role is less established than other MMR genes 4. Biallelic MLH3 variants can cause adenomatous polyposis syndromes 5, and reduced MLH3 expression has been observed in breast cancer 6. Recent studies also implicate MLH3 variants in female infertility related to oocyte and early embryo competence defects 7.