Implication of the oestrogen receptor alpha (ER) in the development of breast cancer is established. High concentration of ER in a primary tumour is a good prognostic factor and, moreover, an index of favourable response to antioestrogens (e.g. tamoxifen) administration. Hence, study of the mechanisms governing ER expression appears of prime importance. This paper overviews accumulated clinical and experimental data concerning the expression of the ER gene, the regulation of both the ER-mRNA and peptide level as well as the acquisition and maintenance of oestrogen binding capacity. Factors playing a role in cell proliferation and/or differentiation of which the expression is positively or negatively related to ER level are also evoked, in view of the fact that some of them may be involved in these regulatory processes.
