Kennedy's disease is an X-linked form of motor neuron disease caused by an expanded polyglutamine repeat in the androgen receptor. While the expansion mutation causes some loss of transcriptional activity by the androgen receptor, the predominant effect of expansion is probably a toxic gain of function, similar to the mechanism of other polyglutamine expansion diseases. Features of the neurodegenerative phenotype of Kennedy's disease have now been reproduced in transgenic animals and neuronal cell culture. Nuclear inclusions of mutant androgen receptor protein are found in these model systems and in autopsy samples from patients with Kennedy's disease.