BackgroundInterferon-β (IFNβ) has proven to be an important advance in the therapy of multiple sclerosis (MS), but optimal markers for bioactivity have not been identified. To accurately measure bioactivity in MS patients treated with IFNβ, we developed and tested a real-time reverse transcriptase (RT)-PCR assay for gene expression ofMxA, an IFNβ-induced gene in the peripheral blood of patients treated with IFNβ.MethodsWe compared IFNβ-treated patients with MS to controls in expression ofMxArelative to the house-keeping gene,GAPDH. 2′-5′oligoadenylate synthetase (OAS) gene expression was also tested by real-time RT-PCR on RNA from the same patient specimens. Anti-IFNβ antibody was measured by ELISA and a cytopathic effect assay.ResultsSeven of 54 patients were found to have complete loss of bioactivity.MxAexpression correlated well withOASexpression. All patients with lost bioactivity had high levels of binding antibodies or neutralizing antibodies.ConclusionsThis is the first demonstration that a real-time RT-PCR assay can be used to monitor therapy with interferons. These data identifyMxAmRNA as an excellent biomarker for INFβ action on the IFN receptor, and clarify the relationship between anti-IFNβ antibodies and bioactivity in patients with MS treated with IFNβ.