**1. Introduction**

64 Steroids – Basic Science

Wu, Y.L., Yang, X., Ren, Z., McDonnell, D.P., Norris, J.D., Willson, T.M. & Greene, G.L.

Cell 2005, 18:413–424.

(2005*) Structural basis for an unexpected mode of SERM-mediated ER antagonism.* Mol

Discussions on receptors involved in estrogen action(Stanišić et al,2010) have so far focused on the two major forms of "classical" estrogen receptors, the estrogen receptor α (ER α) and estrogen receptor β (ER β). Both are DNA binding as well as hormone binding forms, with distinct, well-characterized functional domains. The differences between the two have mainly been in their respective molecular masses and shapes and in the target genes with which they interacted.

I have been involved in research in estrogen action for 4 decades and more. Since the focus of my work was chiefly on non-conventional estrogen receptors and receptor associated proteins, often the progress made was felt by me as slow.Nevertheless,it is my sincere belief that what has been unveiled in this direction over the years have not gone unproductive. The two proteins that have been identified in this context, one a non DNA binding estrogen receptor and the other a transcription factor that dimerises with this receptor in the nucleus, have pointed towards the existence of a unique system of receptor in estrogen action. For the first time ever, it has become clear that there is a form of estrogen receptor whose primary functional role is in post transcriptional regulatory mechanisms that include splicing, nucleocytoplasmic transport of RNA and finally, the translation of mRNA. Also, deeper insights into the functional biology of the transcription factor have unfolded certain experimental data hitherto unknown in the literature on steroid hormone action. What is being discussed in this chapter deals exclusively with these two proteins, one a plasma membrane localized estrogen receptor which moves into the nucleus to involve itself in gene regulatory events and the other a transcription factor with a parallel functional role in mitochondrial steroidogenesis
