Pathological endpoints such as tumour size, lymph node status and vascular invasion remain the most useful guides in selecting treatment strategies for breast cancer. There is a need, however, to further investigate the molecular mechanisms that determine the properties of an individual tumour e.g., hormone responsiveness and probability of metastasis. While numerous prognostic factors have now been identified few have contributed to defining clinical response to therapy. Oestrogen-regulated genes are likely to be important since they not only define a functional oestrogen receptor, but alterations in their expression might provide insights into the mechanisms involved in tumour progression and loss of endocrine sensitivity. Recently an oestrogen responsive gene, pLIV1, has been isolated and shown to be expressed in ER+ disease where it appears to predict nodal involvement. The present paper describes aspects of its regulation and discusses the potential role of this and other genes in the development of endocrine resistance.