Estrogen inhibiting drugs are commonly used in estrogen receptor positive breast cancers. These include Tamoxifen, Fulvestrant, Letrozole and Anastrozole. In an article published by Svoronos, et al. in the journal Cancer Discovery, January 2017, titled “Tumor Cell-Independent Estrogen Signaling Drives Disease Progression Through Mobilization of Myeloid- Derived Suppressor Cells” they describe how estrogen inhibits anti-cancer immunity irrespective of the receptor status of the cancer. It is known that there are inherent differences in the immune systems of men and women, particularly related to hormonal differences. Estrogen receptors are not only expressed on some tumors, but are also expressed on immune cells. Studies are indicating that estrogen causes an increase in regulatory cells, such as MDSCs, along with reducing infiltration of CD8 positive, tumor-attacking T cells in the tumor microenvironment. The basic idea is that estrogen inhibiting drugs seem to have positive effects on the anti-cancer immune response, no matter if the cancer cells are estrogen receptor positive or not.