Due to conflicting results, the failure of validation of the anti-JCV antibody index, as a tool to stratify PML risk during treatment with ocrelizumab, and the requirement to identify and validate biomarker predictive of a severe adverse event such as PML, in this study previous data were deepened regarding the assessment of anti-JCV antibody index in MS patients, during one year of ocrelizumab treatment, also considering the ocrelizumab’s effects on JCPyV pathogenicity in terms of viral replication and NCCR behavior. This evidence concerns the gene PML and myeloid sarcoma.