B7-H1 expressed on APC acts as a strong inhibitor of autologous CD4 T-cell activation and may thus contribute to the maintenance of peripheral immune tolerance. IFN-β up-regulates B7-H1 in vitro and in MS patients in vivo and might represent a novel mechanism how IFN-β acts as a negative modulator on APC T-cell interactions in the periphery. This evidence concerns the gene CD4 and myeloid sarcoma.