Table 1

Clinical significance of huntingtin CAG repeat lengths

CAG repeat length	Clinical significance
≤26	Normal
27–35	Intermediate—not pathological, but may cause disease through: Expansion into pathogenic range in successive generations Genomic instability resulting in mosaicism with expansion into pathogenic range in some neuronal populations, leading to clinical features of HD in rare cases
36–39	Pathogenic with incomplete penetrance
≥40	Pathogenic with complete penetrance