In trinucleotide repeat disorders, where as the length of the number of trinucleotide repeats increases, so too does the likelihood of further repetitive mutation (a phenomenon called dynamic mutation), the pre-mutation range is the range at which a carrier has enough repeats not to show symptoms of the disease, but such that there is a significant probability of further mutation, putting their offspring at risk.

Although generally pre-mutation carriers are asymptomatic, they may occasionally develop (especially in later life) a disease that is closely related to the disease caused by full mutation. For instance, some men and women with CGG pre-mutation do not have Fragile X Syndrome, but do have Fragile X-associated tremor (or ataxia syndrome), which is characterised by movement difficulties (ataxia), memory loss, peripheral neuropathy (loss of sensation in the extremities) and a range of mental and behavioural changes.