In the last two decades, several studies have been carried out to identify sensitive and predictive markers detectable to prevent PML disease, including L-selectin (CD62L), C-reactive protein or agnoprotein detection, JCPyV serostatus, viremia, viruria and the onset of JCPyV NCCR variants. The gene discussed is SELL; the disease is progressive multifocal leukoencephalopathy.