This letter, published in the Journal of Neurology, discusses the case of a patient with multiple sclerosis (MS) who developed progressive multifocal leukoencephalopathy (PML) while being treated with fingolimod. PML is a central nervous system infection caused by the reactivation of the John Cunningham virus (JCV), and it is a concern for MS patients treated with certain medications. The patient's PML progressed despite treatment discontinuation and therapeutic plasma exchanges, but they were successfully treated with three cycles of pembrolizumab, an immune checkpoint inhibitor. The use of pembrolizumab for PML in MS patients is still being studied, but it may be a potential treatment option to stop the progression of the disease. [Extracted from the article]