Kailash Bhatia, MD, DM, FRCP, UCL Queen Square Institute of Neurology, National Hospital for Neurology and Neurosurgery, London, UK, discusses how extrinsic factors, such as environmental factors, contribute to the variable expression of movement disorders. Prof. Bhatia shares the example of patients with DYT1 dystonia, where subjects carrying the DYT1 mutation can have early-onset severe dystonia, mild dystonia, or even carry the gene but not manifest dystonia. Although the exact causes are not known, it is hypothesized that exposure to certain environmental factors can precipitate dystonia. In the future, case-control studies are needed to identify environmental factors and understand their role in the expression of genetic forms of dystonia and other movement disorders. This interview took place during the XXV World Congress of Neurology.