PolyQ disorders exhibit certain common clinical and pathological features despite the fact that their single common genetic attribute is the pathogenic CAG repeat expansion and their affected genes are otherwise unrelated (Table ). PolyQ diseases are all neurodegenerative disorders, with onset usually in midlife and slowly progressing phenotypes. These diseases demonstrate preferential degeneration of distinct cell types. The age of onset is inversely proportional to the CAG tract length, with this relationship being complex. Less common cases of childhood-onset disease occur in children with very long repeats. Furthermore, the expanded repeat is unstable and may increase in length from one generation to the next, wherein disease manifestations occur at earlier ages in subsequent generations. Expanded polyQ mutations are believed to cause cellular toxicity as a result of misfolded mutant proteins. All polyQ diseases except for SBMA are inherited in an autosomal dominant way, while SBMA is caused by a CAG repeat expansion in the X-linked androgen receptor gene, with the disease occurring only in men.