Elevated levels of C4a and C4b have been reported in the CSF of persons with relapsing-remitting MS compared with healthy controls, with elevated levels for C4a in those with active vs stable disease.55C4 has also been associated with the Expanded Disability Status Scale in MS.55 Considering these relationships, complement system activation may be a key mechanism that underlies the associations between AAO and MS outcomes. The gene discussed is C4A; the disease is relapsing-remitting multiple sclerosis.