Moreover, L-selectin seems to be an ambiguous biomarker in natalizumab-associated PML MS patients whereas matrix metalloproteinase 9 decrease may be a risk for PML in the same group, and also, serum neurofilaments have been reported to be elevated in pre-PML and PML in MS cases [94,95,96,97]. The gene discussed is SELL; the disease is progressive multifocal leukoencephalopathy.