Sequences of three nucleotides (the basic constituent of DNA) repeated in tandem on the same chromosome a number of times. A normal variation in repeat number with no clinical significance commonly occurs between individuals. However, repeat numbers over a certain threshold can, in some cases, lead to adverse effects on the function of the gene and result in genetic disease.
Trinucleotide repeats are the underlying genetic mechanism in diseases which show "anticipation" - please see the entry on this topic.
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