At least five adult–onset neurodegenerative diseases, including Huntington disease (HD), and dentatorubral–pallidoluysian atrophy (DRPLA) are produced by genes containing a variably increased CAC repeat within the coding region^1–4. The size range of the repeats is similar in all diseases; unaffected individuals have fewer than 30 CAG repeats, whereas affected patients usually have more than 40 repeats. The size of the inherited CAG repeat correlates with the severity and age of disease onset^1,5–7. The CAG triplet repeat produces a polyglutamine domain in the expressed proteins^3,8–10. All of these diseases are inherited in a dominant fashion, and a pathologic gain of function in gene carriers has been proposed. We sought to identify proteins in the brain that selectively interact with polyglutamine–domain proteins, hypothesizing that the polyglutamine domain may determine protein–protein interactions.