Nucleotide excision repair, not to be confused with base excision repair, involves the removal and replacement of damaged nucleotides. Instances of crosslinking, adducts, and lesions (generated by ultraviolet light or reactive oxygen species) can trigger this repair pathway. Short stretches of single stranded DNA containing such damaged nucleotide are removed from duplex DNA by separate endonucleases effecting nicks upstream and downstream of the damage. Deletions or mutations which affect these nucleases instigate increased sensitivity to ultraviolet damage and carcinogenesis. Such abnormalities can even impinge neural development.