Even repeats of 27-35 CAGs can expand occasionally; these are called " pre-mutation" alleles, since they don't cause the disease, but they're set-up for developing a mutation of 36 or more CAGs.

即使CAG序列重复在27-35次间，偶尔也会产生异常，这些被称为突变前等位基因，尽管不会引起疾病，但仍可能会发展为36次以上CAG序列重复的突变。