Table 7 Examples of nucleotide repeat expansion disorders
DiseaseGeneRepeatNormal rangePathogenic rangeDisease featuresEstimated incidence
Huntington diseaseHTTCAG (encoding glutamine)9–3536–39 (possibly pathogenic)
>39 (pathogenic)
Uncontrolled movements, emotional problems, loss of cognitive ability3–7/100000
Myotonic dystrophy type 1DMPKCTG (3′-UTR)5–3750–150 (mildly affected)
100–1000 (classic symptoms)
>2000 (congenital onset)
Progressive muscle wasting and weakness, muscle contractions, cataracts, cardiac abnormalities>1/8000
Fragile X-associated tremor/ataxia syndrome (FXTAS)FMR1CGG (5′-UTR)5–4055–200 (pre-mutation with respect to FXS)Ataxia, tremors, cognitive decline, learning disabilities, blood pressure problems1/4000 males, 1/8000 females (milder in females)
Fragile X syndrome (FXS)FMR1CGG (5′-UTR)5–40200-several thousandDevelopmental problems including learning disabilities and intellectual impairment, autistic spectrum disorders, attention deficit
Friedreich ataxiaFXNGAA (in intron 1)5–3366 to >1000Impaired muscle coordination, loss of strength and sensation, muscle stiffness, impaired speech, hearing, and vision, heart disease1/40000 in people of European, Middle Eastern or North African ancestry
  • Four genes subject to repeat expansions are shown, with the gene affected, repeat sequence, normal and pathogenic range of repeat, main disease features and estimated incidence.