Our research involving TrkbPenk-KO mice demonstrates that both the striatum and the cerebellum may be dysfunctional as a consequence of reduced BDNF–TrkB signalling in specific neuronal types in these brain areas, raising the possibility that a common pathway is responsible for some of the motor symptoms present in polyQ disorders such as HD, SCA1, and SCA6. This evidence concerns the gene BDNF and Huntington disease.