Myxovirus A (MxA) is a protein that is specifically induced by treatment with type I cytokines and has proven to be a reliable biomarker of interferon-beta (IFNbeta) bioactivity. IFNbeta-induced MxA can be measured as either protein or mRNA in the blood of IFNbeta-treated patients with MS. In patients with MS who are treated with IFNbeta, loss of the MxA response is caused by high levels of anti-IFNbeta antibodies and is a sensitive marker of lost bioactivity.