The lymphocytes of one natalizumab-treated patient who developed PML (PML-NAT#2) showed a similar pattern of T- and B-lymphocyte increase during the period of treatment, while in the other one (PML-NAT#1), the increase of most T- and B-cell subsets was followed by their decrease at the time of PML onset. Here, BRD2 is linked to progressive multifocal leukoencephalopathy.