Methods and compositions for determining whether a subject is at risk for PML, including subjects being treated with immunosuppressants, by determining whether the subject harbors a JCV variant with reduced binding for sialic acid relative to a normal JCV, are presented. Furthermore, combinations of JCV-VP1 sequence variations that are associated with PML and that can be used as a basis of an assay for identifying subjects susceptible to PML, subjects with PML (e.g., early stage PML), or subjects at risk of developing PML in response to an immunosuppressive treatment are provided.