The fact that dimethyl fumarate-associated PML typically occurs in the context of severe lymphopenia, whereas abnormal production of IL-10, expression of PD-1, and reduced expression of CD49d by JCV-specific T cells are observed in natalizumab-induced PML, suggests that not only the number of lymphocytes but also their composition is important in the pathogenesis of PML30–32. The gene discussed is IL10; the disease is lymphopenia.