In order to assess whether PML could be explained by a dysregulated immune system caused by systemic sarcoidosis, we analysed the immune cell composition in the peripheral blood by flow cytometry, which revealed decreased numbers of CD4+ T cells (139.9 μl−1; reference: 500–1350 μl−1), and an increased frequency of Tregs (13% of CD4+ T cells; mean of 10 healthy controls 6.1%, SEM ± 1.007). This evidence concerns the gene CD4 and progressive multifocal leukoencephalopathy.