Endonuclease is a class of enzymes capable of cleaving phosphodiester bonds within nucleic acid chains. This family includes various enzymes such as restriction endonucleases, repair endonucleases, and ribonuclease H. Endonucleases are primarily located in the nucleus but can also be found in the cytoplasm and mitochondria. They play a central role in DNA repair, recombination, transcriptional regulation, and RNA processing. The activity and expression of endonucleases are tightly controlled by the cell. Genetic mutations can lead to dysfunction in repair mechanisms, increasing the risk of hereditary diseases and cancer.