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An array of tandem repeats is defined as several (at least two) adjacent copies having the same or similar sequence motifs. These periodic sequences are generated by internal duplications in both coding and non-coding genomic sequences. Repetitive units of protein tandem repeats are considerably diverse, ranging from the repetition of a single amino acid to domains of 100 or more residues.



Distribution in proteomes
Tandem repeats are ubiquitous in proteomes and occur in at least 14% of all proteins. For example, they are present in almost every third human protein and even in every second protein from Plasmodium falciparum or Dictyostelium discoideum. Tandem repeats with short repetitive units (especially homorepeats) are more frequent than others.

Structure of tandem repeats
Approximately half of the tandem repeat regions has intrinsically disordered conformation being naturally unfolded. The other half of the regions with the stable 3D structure has a plethora of shapes and functions. Depending on the length of the repetitive units, their protein structures can be subdivided into five classes:


 * 1) crystalline aggregates formed by regions with 1 or 2 residue long repeats
 * 2) fibrous structures stabilized by inter-chain interactions with 3-7 residue repeats
 * 3) structures with the repeats of 5–40 residues dominated by solenoid proteins
 * 4) closed (not elongated) structures with the repeats of 30-60 residue long
 * 5) beads on a string structures with typical size of repeats over 50 residues, which are already large enough to fold independently into stable domains.

Functions of proteins with tandem repeats
Some well-known examples of proteins with tandem repeats are collagen, which plays a key role in the arrangement of the extracellular matrix; alpha-helical coiled coils having structural and oligomerization functions; leucine-rich repeat proteins, which specifically bind a number of globular proteins by their concave surfaces; and zinc-finger proteins, which regulate the expression of genes by binding DNA.