Several drugs currently used in the treatment of MS—including dimethyl fumarate, the sphingosine-1-phosphate receptor modulator fingolimod, the anti–α4-integrin antibody natalizumab, and the anti-CD20 antibody ofatumumab—are associated with an increased risk for progressive multifocal leukoencephalopathy (34). Here, S1PR1 is linked to myeloid sarcoma.