This is my first book about functions of estrogen receptors from molecular and physiological point of views. This issue has been selected because I think that estrogen has different dimensions out the scope of reproductive matters. From a philosophical point of view, its presence in males and females represents a phenomenon that deserves studying. Most severe cases of pathology, particularly lymphoma were more likely to be among males who exhibited strong expression estrogen receptor. From a molecular point of view, it can induce cancer, and offers protection for central nervous system. From evolutionary point of view, we demonstrated the expression of estrogen receptor and BCL2 proteins in Candida Albicans, which may indicate that as the fungi grow more, as a model for cancer is developed. Moreover, the barriers between prokaryotic and eukaryotic cells may be removed from a functional point of view, and the interactions between cells take a deeper level.