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. Author manuscript; available in PMC: 2019 Apr 26.
Published in final edited form as: Handb Clin Neurol. 2018;147:105–123. doi: 10.1016/B978-0-444-63233-3.00009-9

Figure 3. Normal and disease repeat lengths for the CAG/polyglutamine diseases.

Figure 3.

Normal (green), incompletely penetrant (yellow) and fully penetrant (red) repeat lengths are shown for each disease. Deepening color of red illustrates increased disease severity and earlier age of onset with longer repeat lengths. Arrows illustrate that the longest disease repeats in spinocerebellar ataxia type 2 (SCA2), SCA7 and Huntington disease are >100 repeats. Gray regions represent size range of intermediate length repeats that may be prone to further expansion. High normal SCA2 alleles (orange) are a risk factor for ALS. At SCA1 locus, normal repeats can be longer when they are interrupted by CAT residues. DRPLA, dentatorubral-palliodoluysian atrophy; SBMA, spinobulbar muscular atrophy

* Normal repeat length in SCA1 and SCA6 are modifiers of age of onset, and SCA6 normal repeat length is also a modifier of age of onset in SCA2.

# In SCA2 disease repeats interrupted by CAA can be associated with dominantly inherited Parkinson disease rather than ataxia.

^ Normal repeat in SCA17 is an imperfect CAG repeat interrupted with CAA residues.