Retinoids mediate their actions via retinoic acid receptors（RARs） and retinoid X receptors（RXRs）.Each of class of these nuclear retinoid receptor is further subdivided into three species namelyα

βandγ.Recently studies demonstrated that estrogen receptor（ER）-positive human breast cancer（HBC） cell lines are sensitive and ER-negative cell lines are resistant to growth inhibitory effects of retinoic acid（RA）.In this study

we found that only RARαmRNA levels was strongly correlated with ER-status.To further investigate the major role of RARαin retinoid-mediated inhibition of growth

we transfected RARαcDNA into two RA-resistant ER-negative HBC cell lines.Analysis of different clonal populations of RARαtransfectants from each cell line revealed growth inhibition by retinoids.Our results demonstrated that RARαplays a major role in mediating retinoids inhibition of growth in HBC cells and adequate levels of RARαare required for such an effect.