Using flow cytometry, in a blinded study, we measured plasma CD31+EMP in 30 consecutive patients with relapsing-remitting MS (RRMS) prior to and 4, 12, 24 and 52 weeks after initiation of intramuscular therapy with interferon-β1a (IFN-β1a), 30 micrograms weekly. The gene discussed is PECAM1; the disease is relapsing-remitting multiple sclerosis.