Transposable elements have clearly played an important role in shaping the genomes of many organisms. The large size of many eukaryotic genomes is due primarily to the abundance of transposable elements, particularly retrotransposons. Homologous recombination between copies of transposable elements has been an important force in producing gene duplications and other DNA rearrangements. Furthermore, some transposable elements may carry extra DNA with them when they transpose to a new site, providing the potential to move DNA sequences that regulate genes to new sites, where they may alter the expression of genes.