IFNB1 and progressive multifocal leukoencephalopathy: Despite a number of interesting aspects to the immunopathogenesis being identified as important in MS, with the exception of neutralizing antibodies which can explain breakthrough disease in some patients (IFNβ and natalizumab) and the JC virus antibody test which can stratify risk of PML in the use of natalizumab, there are currently no clinically useful predictive biomarkers to guide individualized therapy in MS.