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Inclusion Criteria:

* Adults (≥18 years old) with a diagnosis of relapsing forms of multiple sclerosis (RMS) or primary progressive multiple sclerosis (PPMS) according to the 2017 revised McDonald criteria.
* Previous MS treatment with at least one of other DMT(\*). The patients can be without treatment before switching until the end of wash-out period of previous DMT(s) or until lymphocytes parameter is in normal range.
* Previous treatment change with the reasons inefficacy, safety related issues or lack of compliance.
* Decision to initiate ocrelizumab therapy (in accordance with the product characteristics approved in Turkey) has already been taken for the treatment of MS patient as part of routine clinical practice. The decision to treat with Ocrelizumab must be made prior to and independently from the proposal to enroll the patient into this study.
* Agreed and signed informed consent.

(\*) A DMT is defined as any of the following drugs: Teriflunomide, Interferon beta 1a, Interferon beta 1b, Peginterferon beta 1a, Glatiramer acetate, Fingolimod, Daclizumab, Alemtuzumab, Cladribine, Dimethyl fumarate, and Natalizumab.

Exclusion Criteria:

* Previously treated with anti-CD20 therapy (rituximab, atacicept, belimumab or ofatumumab).
* Medical history of a malignancy, active infection (including Hepatitis B virus) or chronic inflammatory disease.
* Medical history or use of any medication other than a DMT as defined above which may affect immunophenotypes of the participants.

• Istanbul, Uskudar, Turkey (Türkiye)