Genomic instability is a mechanism in which a high frequency of mutation occurs inside the genome of a cellular lineage. These mutations include changes in DNA and RNA sequences, chromosomal rearrangements (aneuploidy). One of the sources of genomic stability is the high frequency of externally caused DNA damage since DNA damages can cause inaccurate translesion synthesis past the damages or errors in repair, leading to mutation. Another source of genome instability may be mutational reductions in expression of DNA repair genes. DNA repair is likely an important source of genome instability. The procedure of hereditary instability prompts aneuploidy, in which the cells introduce a chromosomic number that is either higher or lower than the normal complement for the species.