Steroid receptor specificity with reference to the estrogen receptor

A variety of mechanisms are used by individual cells to modulate the response of specific genes to steroid hormones. In addition to the classical presence or absence of a steroid nuclear receptor, there are several mechanisms for determining tissue specificity: (i) changes in the receptor structure induced by different ligands, (ii) the role of specific transcription factors, (iii) the presence of alternate steroid hormone DNA binding sites which preferentially interact with some but not all members of a particular class, and (iv) the presence of extra receptor enzyme systems which can modify the steroid before it can interact with its nuclear receptor. All these mechanisms participate in turning a potentially homogeneous response system into a very heterogeneous one with a high degree of tissue and substrate specificity. Most of these mechanisms are involved in modifying the reponse of an individual tissue to estrogen agonists and antagonists.