"Living cells respond to DNA damage by a variety of mechanisms, including a series of biochemical pathways called DNA repair. These include three discrete pathways for the excision of damaged bases, called base excision repair, mismatch repair and nucleotide excision repair (NER). NER in human cells is a complex biochemical process during which a large multiprotein complex is assembled at several types of base damage. This multiprotein complex (NER machine) catalyses the excision of damaged bases as oligonucleotide fragments."