Transposable elements—DNA sequences that can move about in the genome—are often a cause of mutations. They are found in the genomes of all organisms and are abundant in many: for example, they make up at least 45% of human DNA. Most transposable elements are able to insert themselves at many different locations in the genome, relying on mechanisms that are distinct from homologous recombination. They often cause mutations, either by inserting into a gene and disrupting it or by promoting DNA rearrangements such as deletions, duplications, and inversions (see Chapter 6).