Abstract: JC virus (JCV) is a human polyomavirus that infects the majority of people without apparent symptoms in healthy subjects. A neuropathogenic JCV variant is the causative agent of progressive multifocal leucoencephalopathy (PML), a disorder following lytic infection of oligodendrocytes that mainly manifests itself under immunosuppressive conditions. A hallmark for JCV isolated from PML-brain is the presence of rearrangements in the non-coding control region (NCCR) interspersed between the early and late genes on the viral genome. Such rearrangements are believed to originate from the archetype JC virus variant which is shed in urine by healthy subjects and PML patients. Next generation sequencing (pyro-sequencing) has been performed to explore the NCCR variability in urine of healthy subjects in search for JCV quasispecies and rearrangements reminiscent of PML.