APA Dictionary of Psychology

n. a specific type of genetic variant in which a DNA sequence, typically a trinucleotide repeat expansion, is larger than normal but below the threshold for a full mutation that results in a genetic disorder. Premutations are typically unstable and may expand to a full mutation in offspring and future generations. An example of a premutation is huntingtin (HTT), the gene implicated in Huntington's disease, in which intermediate or reduced-penetrance cytosine–adenine–guanine (CAG) repeat expansions may not cause classic disease but confer increased risk of neuropsychiatric and motor symptoms and can expand into the pathogenic range of full mutation in subsequent generations.