['Progressive multifocal leukoencephalopathy (PML) is associated with natalizumab treatment in patients with multiple sclerosis (MS). It has been hypothesized that natalizumab mobilizes JC virus (JCV)-infected haematopoietic progenitor cells mediating viraemia and subsequently this disease.', 'The objective of this study was to investigate peripheral haematopoietic progenitor cells for evidence of JCV DNA in MS patients treated with natalizumab.', 'We assessed JCV and cytomegalovirus (CMV) DNA in magnetically separated CD34+ haematopoietic progenitor cells, peripheral blood mononuclear cells and plasma of 67 natalizumab-treated patients with MS and six PML patients.', 'Viral DNA was not detectable in CD34+ haematopoietic progenitor or peripheral blood mononuclear cells from any sample. Two plasma samples from patients with MS while undergoing natalizumab treatment were JCV-positive. In one case clinically manifest PML developed 8 months thereafter.'] Do cD34+ progenitor cells mobilized by natalizumab are not a relevant reservoir for JC virus?