The gene for spinocerebellar ataxia 7 (SCA7) has been mapped to chromosome 3p12-13. By positional cloning, we have identified a new gene of unknown function containing a CAG repeat that is expanded in SCA7 patients. On mutated alleles, CAG repeat size is highly variable, ranging from 38 to 130 repeats, whereas on normal alleles it ranges from 7 to 17 repeats. Gonadal instability in SCA7 is greater than that observed in any of the seven known neuro-degenerative diseases caused by translated CAG repeat expansions, and is markedly associated with paternal transmissions. SCA7 is the first such disorder in which the degenerative process also affects the retina.