DNA whose base sequence is repeated many times throughout the genome of an organism. It is common in eukaryotes, accounting for about half of the total DNA in mammals, for example, and can be divided into various types. Some is involved in maintaining the structure of the chromosomes, but a significant proportion remains of uncertain function, and is probably junk DNA. Highly repetitive DNA typically consists of short (usually <100 bp, but sometimes up to 500 bp) sequences that are repeated hundreds or thousands of times in tandem at any one site—simple sequence DNA. It includes the short tandem repeats (STRs), which form the heterochromatin flanking the centromere and are used as genetic markers in DNA profiling. Tandemly repeated short sequences also occur at each chromosome tip (telomeric DNA). Both types are important for maintaining chromosome structure. Moderately repetitive DNA comprises longer sequences repeated typically 10 to 1000 times and more evenly dispersed throughout the genome. Among these are multiple copies of particular genes or gene sequences; these may be members of gene families or duplicates of genes encoding histones, transfer RNAs, and ribosomal RNAs, which often form tandem arrays. But by far the biggest proportion of repetitive DNA in many eukaryote organisms consists of copies of mobile genetic elements called transposons. In humans, for example, the various types of transposon, notably SINEs and LINEs, account for around 45% of the entire genome. Other distinct types of repetitive DNA lie both in noncoding introns within genes and between genes, where they may act as ‘spacer’ DNA. Among these are variable number tandem repeats (VNTRs), sequences of 5–100 nucleotides repeated hundreds or thousands of times at numerous sites within the genome. Centrifugation of the total DNA separates out repetitive DNA in distinct bands according to its base composition, and hence buoyant density. These bands are thus seen as ‘satellites’ distinct from the band containing the bulk of genomic DNA. There are several categories, including satellite DNA, minisatellite DNA (see variable number tandem repeat), and microsatellite DNA.