There are several cellular machineries that are responsible for repairing the damage to DNA bases. These machineries have specific mechanisms to recognize the damaged DNA bases. The most common form of DNA damage is caused by oxidation, alkylation, or exposure to radiation. When these types of damage occur, it may cause abnormal chemical bonds and structural changes to the DNA molecule. One mechanism for recognizing this damage is through the process of base excision repair (BER), which is initiated by a glycosylase enzyme that recognizes and removes the damaged base from the sugar-phosphate backbone of the DNA. Another mechanism for recognizing DNA damage is through nucleotide excision repair (NER), which is triggered by bulky and helix-distorting DNA damage. The NER machinery identifies such damage and removes the affected DNA segment, which is then replaced with a new sequence. Ultimately, the recognition of DNA damage is based on the altered chemical structure of the DNA that results from the damage. The cellular machinery involved in DNA repair recognizes and responds to these altered structures in order to initiate the repair process.