 The spinocerebellar ataxias SCAs are a clinically and genetically heterogeneous group of autosomal dominantly inherited progressive ataxia disorders. It is estimated that there are 30,000 individuals in the European Community that directly descend from individuals affected by a SCA disorder and thus carry a 50 risk of having inherited an SCA mutation. These at risk individuals provide a unique research opportunity to prospectively study the presymptomatic phase of SCA disorders and to identify the earliest and most sensitive clinical signs and biological markers that herald the onset of the illness. This information is of critical importance for the development of future therapeutic interventions aimed at postponing the clinical onset of ataxia. 