The autosomal dominant (AD) ataxias are a group of disorders affecting the cerebellum and its afferent and efferent pathways. Their pattern of inheritance can be explained on the basis of mutation in a single gene. The trait controlled by this gene is deemed dominant if it manifests in the heterozygote – an AD ataxia is caused by a mutant allele that produces the ataxic phenotype in the affected individual, despite the presence of a normal allele on the second homologous chromosome. This can occur in a number of different ways.