Genetic Evaluation of Natalizumab-Treated Patients With Progressive Multifocal Leukoencephalopathy

The purpose of this study is to explore host genetic mutations which may render individual subjects more susceptible (or resistant) to developing Progressive Multifocal Leukoencephalopathy (PML). Samples will also be collected to determine Deoxyribonucleic Acid (DNA) sequence of JC Virus (JCV). Analysis of the JC Virus (JCV) genome may provide information about viral genotypes that may be associated with higher pathogenicity, and help to identify individuals who may be at higher risk of developing Progressive Multifocal Leukoencephalopathy (PML) due to chronic infection with a more pathogenic variant of JC Virus (JCV).