New perspectives into the biological and clinical relevance of oestrogen receptors in the human breast

Oestrogen receptor (ER) is arguably the single most important biological predictive factor that exists today. In the last 10 years or so, however, our understanding of ER biology has undergone a paradigm shift following the identification of a second ER, ERbeta, with the original ER being renamed ERalpha. A number of isoforms have additionally been described, especially for ERbeta. Our knowledge of ER signalling has also increased with the recognition of accessory co-regulatory proteins, which help direct the transcriptional cascade. Here we outline these changes and discuss what biological and clinical implications these could have in the mammary gland.