Transposable elements, originally discovered in maize by Barbara McClintock, are discrete DNA segments that can insert into new chromosomal locations either by a “cut and paste” or by a “copy and paste” mechanism. Transposable elements are involved in a wide variety of biological transactions including genome alteration by element insertion or deletion and homologous recombination between element copies. They fall into two classes according to whether their transposition intermediate is RNA (class 1 or retrotransposons) or DNA (class 2 or DNA-based transposons). Recent genome sequencing projects have revealed that transposable elements make up a major portion of most vertebrate genomes. For example, transposable elements account for almost 50% of the human genome. This table summarizes different types of transposable element content in human, mouse, opossum, platypus, chicken, and fugu.