These conditions include:
- spinobulbar muscular atrophy (Kennedy's disease)
- Huntington's disease
- spinocerebellar ataxia type 1
- spinocerebellar ataxia type 2
- spinocerebellar ataxia type 3 (Machado-Joseph disease)
- dentatorubral-pallidoluysian atrophy
- Friedreich's ataxia
Typically the disorders are characterized by:
- amplification of the CAG repeat
- the CAG's are translated into tracts of glutamines
- small numbers of repeats in affected individuals, usually <100
- adult onset
- genetic anticipation is not as marked as in the mental retardation group; nevertheless, larger numbers of repeats result in early onset and severe disease
Friedreich's ataxia is exceptional:
- a GAA trinucleotide is expanded
- there is no clear genetic anticipation
- inheritance is autosomal recessive
- onset is earlier, typically in adolescence