DNA mismatch repair (MMR) is a highly conserved pathway that maintains genomic stability by correcting base-base mismatches and insertion/deletion mispairs during DNA replication and recombination. Key proteins in MMR include MutS, MutL, and MutH in *E. coli*, which are homologs in eukaryotic cells. MMR also suppresses homeologous recombination and plays a role in DNA damage signaling. Defects in MMR are linked to genome instability, cancer predisposition, drug resistance, and meiotic abnormalities. Human MMR proteins, such as hMSH2, hMSH6, hMLH1, and hPMS2, are essential for mismatch recognition and repair. MMR is involved in DNA damage response, cell cycle arrest, and apoptosis. MMR deficiency is associated with hereditary non-polyposis colorectal cancer (HNPCC) and other cancers. MMR also plays roles in DNA recombination, repair of interstrand crosslinks, immunoglobulin class switching, somatic hypermutation, and trinucleotide repeat expansion. MMR defects can lead to cancer development and are linked to MSI in various cancers. MMR is crucial for genomic stability and cancer prevention. MMR deficiency can result in resistance to certain chemotherapeutic agents and may contribute to secondary cancers. Understanding MMR mechanisms is essential for developing new therapeutic strategies for cancer patients.DNA mismatch repair (MMR) is a highly conserved pathway that maintains genomic stability by correcting base-base mismatches and insertion/deletion mispairs during DNA replication and recombination. Key proteins in MMR include MutS, MutL, and MutH in *E. coli*, which are homologs in eukaryotic cells. MMR also suppresses homeologous recombination and plays a role in DNA damage signaling. Defects in MMR are linked to genome instability, cancer predisposition, drug resistance, and meiotic abnormalities. Human MMR proteins, such as hMSH2, hMSH6, hMLH1, and hPMS2, are essential for mismatch recognition and repair. MMR is involved in DNA damage response, cell cycle arrest, and apoptosis. MMR deficiency is associated with hereditary non-polyposis colorectal cancer (HNPCC) and other cancers. MMR also plays roles in DNA recombination, repair of interstrand crosslinks, immunoglobulin class switching, somatic hypermutation, and trinucleotide repeat expansion. MMR defects can lead to cancer development and are linked to MSI in various cancers. MMR is crucial for genomic stability and cancer prevention. MMR deficiency can result in resistance to certain chemotherapeutic agents and may contribute to secondary cancers. Understanding MMR mechanisms is essential for developing new therapeutic strategies for cancer patients.