 In cancer, genome instability can occur prior to or as a consequence of transformation.Genome instability can refer to the accumulation of extra copies of DNA or chromosomes, chromosomal translocations, chromosomal inversions, chromosome deletions, single-strand breaks in DNA, double-strand breaks in DNA, the intercalation of foreign substances into the DNA double helix, or any abnormal changes in DNA tertiary structure that can cause either the loss of DNA, or the misexpression of genes.Situations of genome instability (as well as aneuploidy) are common in cancer cells, and they are considered a "hallmark" for these cells.