Our expertise as center for the determination of NAbs has allowed us to evaluate its cross-reactivity in an important cohort of MS patients, not only by inhibition of the antiviral activity of IFNβ by bioassay but also through evaluation of the interference with the JAK-STAT signaling pathway, which is the pathway through which IFNß exerts its function, by phosphoflow cytometry. Here, IFNB1 is linked to myeloid sarcoma.