They also are instrumental in rejection of foreign tissue, and may be important in the body's defense against cancer. Transposon A mobile genetic element that can move from one location in the gene and reinsert at another site. The agent used to carry new DNA into a cell. Viruses or plasmids are often used as vectors. An infectious agent composed of a single type of nucleic acid, DNA or RNA, enclosed in a coat of protein.
Viruses can multiply only within living cells.