DNA repair mechanisms maintain genome stability by preventing the multiplication of genetic errors, caused by environmental factors and intracellular processes, during cell division. Unrepaired damage may permanently alter genome and cell functions, and even minor changes in DNA strand may initiate malignant transformation of the cell. Up to 25 000 changes in DNA are occur daily in a single, actively dividing, cell, and these changes are continuously repaired. If DNA repair mechanisms are impaired, errors will accumulate into the genome. As numerous factors of different nature can cause genetic errors, and thus several different DNA repair mechanisms are necessary to ensure genomic stability.