DNA damage is an abnormal alteration in the structure of DNA that cannot, itself, be replicated when DNA replicates. In contrast, a mutation is a change in the nucleic acid sequence that can be replicated; hence, a mutation can be inherited from one generation to the next. Damage can occur from chemical addition (adduct), or structural disruption to a base of DNA (creating an abnormal nucleotide or nucleotide fragment), or a break in one or both DNA strands. Such DNA damage may result in mutation. When DNA containing damage is replicated, an incorrect base may be inserted in the new complementary strand as it is being synthesized (see DNA repair § Translesion synthesis). The incorrect insertion in the new strand will occur opposite the damaged site in the template strand, and this incorrect insertion can become a mutation (i.e. a changed base pair) in the next round of replication. Furthermore, double-strand breaks in DNA may be repaired by an inaccurate repair process, non-homologous end joining, which produces mutations. Mutations can ordinarily be avoided if accurate DNA repair systems recognize DNA damage and repair it prior to completion of the next round of replication. At least 169 enzymes are either directly employed in DNA repair or influence DNA repair processes. Of these, 83 are directly employed in the 5 types of DNA repair processes indicated in the chart shown in the article DNA repair.