Mistakes during DNA replication can be recognised and repaired by proofreading enzymes. A mismatched nucleotide may occur at the rate of one per 100,000 base pairs, causing a pause in replication. DNA repair enzymes perform a proofreading function and reduce the error rate to one per billion. Many complex mechanisms have evolved to repair damage and alterations in DNA, which can occur as a result of damage from ultraviolet radiation, X-rays etc. As well as from errors during replication. (Over 50 genes have been found in yeast to be involved in repair mechanisms).