Also known as junk deoxyribonucleic acid (DNA), selfish DNA are areas of DNA that have no apparent function, but are passed on from generation to generation. In some cases, the sequences of selfish DNA are repeated (repetitive DNA) and use the "host" organism as a means for survival (survival machine). This phenomena is documented in the Selfish DNA theory, which states that the eukaryotic organisms carrying the replicating DNA are nothing more than survival machines that allow the DNA to survive and reproduce. This type of DNA is generally repetitive in its composition and it is typical of such regions as spacer DNA and satellite DNA.

Recently, the possibility that selfish DNA includes functional genes that are coded with viable characteristics has been suggested, and the selfish DNA theory has been expanded to the selfish DNA/gene theory. Championed by the British author Richard Dawkins in such books as The Selfish Gene and The Blind Watchmaker, the expanded theory proposes that the "host" organism is not merely a survival machine, but an organism that is affected by the selfish DNA/genes. The selfish DNA is functional, and therefore it can influence its own and its host's survival.