It has been reported that interferon (IFN)-alpha/gamma signal transduction pathway is blocked in several cell lines persistently infected with mumps virus (MV) through decrease of STAT-1 alpha. Expression of the MV structural V protein (MV-V) or C terminal CYS-RICH region of the V protein (MV-Vsp) inhibited the establishment of the antivirus state induced by IFN, but not by expression of the MV-P protein, Suppression of IFN-induced STAT-1 alpha, STAT-2, and IRF-9 (p48) induction was also recognized in the cells transfected with expression vector of the MV-V (pTM-V) or MCV-Vsp (pTM-Vsp) protein, eventhough it was in the absence of the other virus protein. It is supposed that the cystein-rich domain of V protein (Vsp) is involved in the suppression of the IFN signal transduction pathway.