Selfish DNA is a term for sequences of DNA that sensu stricto have two distinct properties:
In his 1976 book The Selfish Gene Richard Dawkins suggested the idea of selfish DNA in reaction to the then fairly new revelation of the large proportion of noncoding DNA in eukaryotic genomes. In 1980, two articles in the journal Nature expanded and discussed the concept. According to one of these articles:
In the purest forms of the concepts, units of genetically functional DNA might be viewed as "replicating entities" that affect their replication by manipulating the physiological activities of the cell that they control; in contrast, units of selfish DNA affect their replication by exploiting existing DNA and DNA-manipulating mechanisms in the cell, notionally without significantly affecting the fitness of the organism in other respects.
Irrespective of the strict definition of selfish DNA, there is no sharp, definitive boundary between the concepts of selfish DNA and genetically functional DNA. Often it also is difficult to establish whether a unit of noncoding DNA is functionally important or not, and if important, in what way. What is more, it is not always easy to distinguish between some instances of selfish DNA and some types of viruses.