FAS and synovial sarcoma: To determine the significance of apoptosis mediated by Fas and tumor necrosis factor receptor I (TNFRI) in virus infection-induced SS lesions, C57BL/6 (B6) background Fas gene-deficient lpr/lpr (B6-lpr/lpr) mice, B6-tnfr knockout (KO) mice, B6-lpr/lpr-tnfrKO mice, and B6-wildtype mice were intraperitoneally infected with mouse CMV (MCMV) [63].