Eukaryotic and prokaryotic cells possess multiple mechanisms to repair DNA and control damage to their genomes. These include base excision repair (BER) and nucleotide excision repair (NER) that excise and replace damaged nucleotide bases and helix-distorting lesions, respectively. Many of the proteins involved in NER are also active in the related transcription-coupled repair (TCR). In addition, mismatch repair (MMR) proteins act to replace mismatched nucleotides and repair insertion/deletion loops. Furthermore, there are two types of double-stranded DNA break repair, homologous recombination (HR) and non-homologous end-joining (NHEJ).