An allele in which a tandemly repeated nucleotide sequence within or near a gene contains more repeats than a normal allele. A premutation allele can expand into a full-penetrance allele (repeat size associated with disease) when passed through the germline. Although premutation alleles are not typically associated with disease, in rare instances they are; the best example is premutation FMR1 alleles, which are associated with disease phenotypes distinct from fragile X syndrome (which is caused by full-penetrance FMR1 alleles).