Table 7. Examples of nucleotide repeat expansion disorders.

Disease	Gene	Repeat	Normal range	Pathogenic range	Disease features	Estimated incidence
Huntington disease	HTT	CAG (encoding glutamine)	9–35	36–39 (possibly pathogenic) >39 (pathogenic)	Uncontrolled movements, emotional problems, loss of cognitive ability	3–7/100000
Myotonic dystrophy type 1	DMPK	CTG (3′-UTR)	5–37	50–150 (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)	FMR1	CGG (5′-UTR)	5–40	55–200 (pre-mutation with respect to FXS)	Ataxia, tremors, cognitive decline, learning disabilities, blood pressure problems	1/4000 males, 1/8000 females (milder in females)
Fragile X syndrome (FXS)	FMR1	CGG (5′-UTR)	5–40	200-several thousand	Developmental problems including learning disabilities and intellectual impairment, autistic spectrum disorders, attention deficit
Friedreich ataxia	FXN	GAA (in intron 1)	5–33	66 to >1000	Impaired muscle coordination, loss of strength and sensation, muscle stiffness, impaired speech, hearing, and vision, heart disease	1/40000 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.