Transposable element
Transposons may have been co-opted by the vertebrate immune system as a means of producing antibody diversity: The V(D)J recombination system operates by a mechanism similar to that of transposons. This is a system of three genes which get rearranged in the production of vertebrate lymphocytes. The system diversely encode proteins to match antigens from bacteria, viruses, parasites, dysfunctional cells such as tumor cells, and pollens.