Sequence of n nucleotides repeated a number of times in tandem; can occur within or near a gene. The size of nucleotide repeats varies: smaller numbers of repeats are common and not associated with phenotypic abnormalities; abnormally large numbers of repeats may be associated with phenotypic abnormalities and are classified as (in increasing order): mutable normal alleles, premutations, reduced-penetrance alleles, and full-penetrance alleles.

Related terms: premutation; trinucleotide repeat