In recent years, advances in molecular genetics have lead to a fundamental shift in our understanding of many neurological disorders. With the completion of the Human Genome Project, a large part of the sequences of all human genes has become known, among them numerous genes in which mutations have been found that can lead to neurological disorders. This new knowledge has enabled the reclassification of many heterogeneous clinical syndromes of unknown etiology on the basis of their molecular defects and their assignment to monogenic subgroups. Analysis of the gene products involved provides insight into molecular pathogenesis and pathophysiology, thus leading to the development of new causal treatment strategies. In practice, this opens new diagnostic possibilities that must be used in a proper and targeted manner (Statement of Practice Committee, 1996; Gasser et al., 2000).