生物细胞中的DNA串行里面包含许多重复串行（repeated sequence），主要可分为两大类，分别是串联重复串行与散在重复串行，主要类型中另外还有更多不同型态：

Repeated sequences (aka. repetitive elements, or repeats) are patterns of nucleic acids (DNA or RNA) that occur in multiple copies throughout the genome. Repetitive DNA was first detected because of its rapid reassociation kinetics.

In many organisms, a significant fraction of the genomic DNA is highly repetitive, with over two-thirds of the sequence consisting of repetitive elements in human. Debates regarding the potential in vivo functions of these elements have been long standing. Controversial references to ‘junk’ or ‘selfish’ DNA were put forward early on, implying that repetitive DNA segments are remainders from past evolution or autonomous self-replicating sequences hacking the cell machinery to proliferate. Repetitive elements found in eukaryotic genomes fall into different classes, depending on their mode of multiplication and/or structure. The disposition of repetitive elements consists either in arrays of tandemly repeated sequences, or in repeats dispersed throughout the genome (see below). Originally discovered by Barbara McClintock, dispersed repeats have been increasingly recognized as a potential source of genetic variation and regulation. Together with these regulatory roles, a structural role of repeated DNA in shaping the 3D folding of genomes has also been proposed. This hypothesis is only supported by a limited set of experimental evidence. For instance in human, mouse and fly, several classes of repetitive elements present a high tendency for co-localization within the nuclear space, suggesting that DNA repeats positions can be used by the cell as a genome folding map.