This study will evaluate the risk of progressive multifocal leukoencephalopathy (PML), a rare brain infection caused by John Cunningham virus (JCV), among people with multiple sclerosis who have been treated with ozanimod. The study will collect and review reported cases of PML to describe patient characteristics and estimate how often PML occurs among people treated with ozanimod. Researchers will assess factors that may be associated with the occurrence of PML, such as age, duration of ozanimod exposure, prior immunosuppressant use, lymphopenia, and JCV antibody status .