Estrogen receptors are contained throughout a number of cells for estrogen and estrogen like substances in the female body. These cells are found throughout the body in organs such as the breast, vagina and ovaries, and estrogen stimulates these cells to divide and replicate. The replication of these cells is unwanted should the person be diagnosed with a cancer such as breast cancer. Tamoxifen has the ability to attach to the estrogen receptors and block the ability of estrogen from binding with these cells, thus preventing replication.

Some women diagnosed with breast cancer are what is known as HER2-positive.  HER2 (human epidermal growth factor-2) is a protein that has bee nshown to promote the growth of cancer cells.  About one-third of all breast cancers make an excess of HER2.  This excess of HER2 can be countered with the drug Herceptin.