Multiple sclerosis is chronic, inflammatory disease of the central nervous system (CNS) characterised by areas of demyelination, or plaques, in the CNS. In 85% of subjects who later develop MS, clinical onset is with an acute or subacute episode of neurological disturbance due to a single white-matter lesion (e.g. optic neuritis, or an isolated brainstem or partial spinal-cord syndrome). This presentation is known as a Clinically Isolated Syndrome (CIS). Because a CIS is typically the earliest clinical expression of MS, research on subjects with a CIS may provide new insights into early pathological changes and pathogenetic mechanisms that might affect the course of the disorder.