The DYT1 gene mutation is associated with early onset generalised dystonia. However, only 30-40 per cent of gene carriers develop symptoms. We have used electrophysiological tests to search for subclinical effects of the presence of the mutation in non-manifesting cases and compared these with those seen in clinically affected cases and healthy controls. Clinically affected patients had the same pattern of abnormalities in spinal and motor cortical circuits as described previously in non-genetically characterised patients. Non-manifesting cases had some but not all of these defects, suggesting that additional genetic or environmental factors may be needed to produce the full range of physiological deficiencies needed to give rise to clinical symptoms.