SELL and progressive multifocal leukoencephalopathy: In the last two decades, many other candidate markers such as L-selectin (CD62L) [11], C-reactive protein or agnoprotein detection [12], T-cell response [13], JCPyV DNA detection in urine (viruria) and blood (viremia) [14,15,16,17] and modifications in the non-coding control region (NCCR) of JCPyV [15] have been proposed to predict the PML risk.