Indeed, recent work has shown that HIV+ PML patients display “blind spots” in their spectrum of neutralizing antibodies [19]: while they may present high neutralizing antibody (NAb) titers against several JCV genotypes, they have no or low NAb titers against JCV PML strains harboring mutations such as L55F or S269F, rendering them vulnerable to these specific strains. This evidence concerns the gene PML and progressive multifocal leukoencephalopathy.