Nucleotide excision repair (NER) is a cellular mechanism that repairs damaged DNA. It specifically targets bulky DNA lesions, such as those caused by ultraviolet (UV) light or chemical exposure. The process begins with the recognition of the damaged site, followed by the removal of a short single-strand segment of DNA that includes the lesion. After the damaged segment is excised, the gap is filled in by DNA polymerase, which synthesizes new DNA using the undamaged strand as a template. Finally, DNA ligase seals the newly synthesized DNA to restore the integrity of the DNA molecule.