Objective To study the expression of tumor suppressor gene p90 ribosomal S6 kinase 4（RSK4） and the interacting protein tumor necrosis factor receptor-associated factor 4（TRAF4） in different human breast cancer cell lines，and to study the potential role of these proteins in breast cancer invasion and metastasis. Methods Real-time fluorescence quantitative PCR （RT-PCR） was used to detect the mRNA expression of TRAF4 and RSK4 in four human breast cancer cell lines （HBL-100，MCF-7，Her-2-positive type，MDA-MB-231）. Western blotting was used to detect the expression of RSK4，TRAF4 and MAPK pathway proteins downstream of ERK1/2 protein in the same four breast cancer lines. Results RSK4 and TRAF4 mRNAs were expressed in all four cell lines；the highest expression was observed for RSK4 mRNA in HBL-100 cells. Expression of RSK4 mRNA was slightly lower in MCF-7，Her-2-positive type and MDA-MB-231 cells. Expression of TRAF4 mRNA followed the trend: HBL-100P<0.05）. The levels of RSK4 and TRAF4 proteins in the four cell lines strongly correlated with the levels of mRNA（r_RSK4=0.92，P<0.01；r_TRAF4=0.82，P<0.01）；the expression of ERK1/2 protein in the four cell lines followed the trend: MDA-MB-231>Her-2-positive type>MCF-7> HBL-100. RSK4 mRNA levels correlated negatively with TRAF4 mRNA levels（r₁=-0.81，P<0.01），and the same was observed for protein levels （r₂=-0.84，P<0.01）. Conclusions RSK4 and TRAF4 expression negatively correlate with each other，which may be mediated by MAPKs downstream of the ERK pathway.