Extrachromosomal DNA (abbreviated ecDNA) is any DNA that is found off the chromosomes, either inside or outside the nucleus of a cell. Most DNA in an individual genome is found in chromosomes contained in the nucleus. Multiple forms of extrachromosomal DNA exist, and, while some of these serve important biological functions, they can also play a role in diseases, such as ecDNA in cancer. In addition to DNA found outside the nucleus in cells, infection of viral genomes also provides an example of extrachromosomal DNA.