Genetic or environmental factors cause several types of DNA damage, including bulky adducts, abasic sites, DNA SSBs, DSBs, base alkylation, methylation, base mismatches, insertions and deletions. These DNA lesions may contribute to neuronal loss and AD progression. The major DNA repair pathways are base excision repair (BER), nucleotide excision repair (NER), mismatch repair (MMR), double strand break repair (DSBR) and direct reversal. DSBR includes homologous recombination (HR) and non-homologous end joining (NHEJ).