CCR4 and progressive multifocal leukoencephalopathy: Lymphopenia represents the main safety concern of DMF treatment due to its association with rare cases of progressive multifocal leukoencephalopathy.17 While it is known that patients with relevant lymphopenia are older and have lower baseline lymphocyte counts – specifically of CCR4+ T helper cells –4,18 more refined biomarkers for risk stratification of this severe side effect are utmost needed to tailor personalized treatment recommendations.