To further investigate the role of DR4 alleles for antibody development, we determined the activity of IFN-β antibodies by another assay, which was used in a number of clinical studies. The in vitro MX1 test was performed on 203 randomly selected samples. As expected, a significant increase (p value = 0.003) of neutralizing in vitro activity was found in sera of HLA-DRB1∗0401- and ∗ 0408-positive subjects (median activity = 8462 NU) compared to sera from HLA-DRB1∗0401- and ∗0408-negative patients (median activity = 0) (data not shown).