AIM:Xaf1-Saos inducible cell lines,which contain "gene switch" system were used to detect the effect of Xaf1 on tumor necrosis factor receptor(TNFR) signal pathway and to investigate the mechanism of cooperation between Xaf1 and TNF-? in inducing cell apoptosis.METHODS:Xaf1 on TNFR1 expression was measured by RT-PCR and Western blotting.The effect of NF-?B on Xaf1 induced apoptosis was detected by DNA content flow cytometry after co-transfection.DNA binding activity of NF-?B was identified by gel mobility shift assay and transcription activity of NF-?B was analyzed by luciferase assay and RT-PCR.SAPK/JNK activity was checked by SAPK/JNK assay.RESULTS:Xaf1 did not modulate TNFR1 at protein and mRNA levels.Increased NF-?B activity in cells inhibited Xaf1 induced apoptosis.Expression of Xaf1 impaired modestly TNF-? induced NF-?B DNA binding activation and transcription activation,also modestly reduced SAPK/JNK activity.CONCLUSION:Xaf1 inhibits TNFR signal pathway,partly contributing to cooperation with TNF-? to induce apoptosis.