The purpose of this study is to compare biomarker levels in Multiple Sclerosis (MS) patients before and after beginning Natalizumab.

Inclusion Criteria:

1. Diagnosis of clinically definite relapsing remitting MS (RRMS)
2. Age 45 years and older
3. Willing and able to provide written informed consent
4. Patient has high disease activity.
5. Patient is about to begin Natalizumab (Tysabri) therapy.

Exclusion Criteria:

1. Any clinically significant disease other than MS that is likely to interfere with the evaluation of CDMS
2. Known infectious or hematological disease.
3. Unwilling or unable to comply with the requirements of this protocol
4. Subject can not have a gadolinium enhanced MRI

The study populations for this study is a patient with a diagnosis of relapsing remitting multiple sclerosis (RRMS) who is about to begin Natalizumab (Tysabri) therapy for a relapse in clinical symptoms (either diagnosed clinically or via gadolinium MRI).