DNA mismatch repair (MMR) is a highly conserved biological pathway crucial for maintaining genomic stability. It primarily corrects base-base mismatches and insertion/deletion mispairs generated during DNA replication and recombination. MMR also suppresses homologous recombination and plays a role in DNA damage signaling in eukaryotic cells. Key components of the MMR pathway include *Escherichia coli* MutS, MutL, and their eukaryotic homologs, MSH2, MSH6, MLH1, and PMS2. These proteins work together to recognize and repair DNA mismatches, with MutS initiating the process, MutL enhancing mismatch recognition and recruiting MutH for cleavage, and other proteins facilitating strand-specific repair. Defects in MMR are associated with genome-wide instability, increased cancer susceptibility, and resistance to certain chemotherapeutic agents. MMR deficiency can lead to microsatellite instability (MSI), which is observed in hereditary non-polyposis colorectal cancer (HNPCC) and some sporadic cancers. MMR also plays a role in DNA damage signaling, promoting cell cycle arrest and apoptosis, and is involved in other DNA metabolic pathways such as homeologous recombination and interstrand crosslink repair. Understanding the mechanisms and functions of MMR is essential for developing new therapeutic approaches for patients with MSI-positive tumors or MMR-deficient cancers.DNA mismatch repair (MMR) is a highly conserved biological pathway crucial for maintaining genomic stability. It primarily corrects base-base mismatches and insertion/deletion mispairs generated during DNA replication and recombination. MMR also suppresses homologous recombination and plays a role in DNA damage signaling in eukaryotic cells. Key components of the MMR pathway include *Escherichia coli* MutS, MutL, and their eukaryotic homologs, MSH2, MSH6, MLH1, and PMS2. These proteins work together to recognize and repair DNA mismatches, with MutS initiating the process, MutL enhancing mismatch recognition and recruiting MutH for cleavage, and other proteins facilitating strand-specific repair. Defects in MMR are associated with genome-wide instability, increased cancer susceptibility, and resistance to certain chemotherapeutic agents. MMR deficiency can lead to microsatellite instability (MSI), which is observed in hereditary non-polyposis colorectal cancer (HNPCC) and some sporadic cancers. MMR also plays a role in DNA damage signaling, promoting cell cycle arrest and apoptosis, and is involved in other DNA metabolic pathways such as homeologous recombination and interstrand crosslink repair. Understanding the mechanisms and functions of MMR is essential for developing new therapeutic approaches for patients with MSI-positive tumors or MMR-deficient cancers.