Inactive disease; RRMS patients who have been treated with monthly infusions of Tysabri for 12 months and have had stable disease for the last 6 months or more and show stable or improving neurological deficits over 3 months on Tysabri.

Active disease; RRMS patients in acute clinical relapse or with active MRI. Blood sample obtained within 30 days of event AND before steroid treatment (or) patients in early disease on or off first line agents with relapse in prior 3 months AND not treated with steroids for at least 2 months.

Patient with a diagnosis of relapsing remitting multiple sclerosis (RRMS) who is about to begin steroid therapy for a relapse in clinical symptoms (either diagnosed clinically or via gadolinium MRI).

A biomarker test for the level of MSDx Complex-1

A difference in the levels of Complex-1 between subjects with controlled, stable MS vs. subjects in acute MS relapse

A decrease in the level of MSDx Complex-1 in response to steroid therapy for a relapse of MS.