Estrogen is a steriod molecule that is produced by both women and men, but is the main sex hormone produced by women. It's job in women is to help maintain development of female characteristics, as well as maintain female sexual characteristics. Since estrogen is a hormone, it is put into the category of signaling molecules. Signaling molecules respond to target tissues, when they find one they bind to cells inside the tissues called estrogen receptors. This allows estrogen molecules to move throughout the body and only affect the proper areas. Two of the main estrogen target tissues are the uterus and breast. Some of the minor target tissues are the liver, brain, heart, and bones. After estrogen enters a cell and binds to the DNA, it activates genes. These genes create messenger RNA which influences the production of the proteins they are responsible for. These proteins affect cell behavior in conjunction with the functions of estrogen molecules. 
According to the National Cancer Institute, the three main functions of estrogen are 1) programming the breast and uterus for sexual reproduction, 2) controlling cholesterol production in ways that limit the buildup of plaque in the coronary arteries, and 3) preserving bone strength by helping to maintain the proper balance between bone buildup and breakdown. 4) Stimulating osteoblast activity that causes rapid bone growth, and eventually cause the epiphyseal places in bones to ossify. Ossification marks the end of bone growth in the human body. 
Estrogen has been described as a sex hormone, secreted by the ovaries (sex organs). The ovaries are also responsible for acting as endocrine glands. These glands produce estrogen and progesterone, and oversee the production of the ovum. Ovulation is the process of replacing the secondary oocyte from a mature follicle. If the follicle is not fertilized, it is transformed into a corpus luteum by the continued secretion of estrogen. Estrogen spikes just before a woman's menstrual cycle, in contrast to many other hormones that are activated during a woman's menstruation such as LH (luteinizing hormone) and FSH (follicle stimulating hormone). (Wile, Shannon. 499-501)
Role in Cancer and Cures
During a woman's menstral cycle, estrogen promotes cell division within the uterus and breast tissue. At the end of each cycle, if a woman does not become pregnant, estrogen levels fall dramatically causing the new cells to die. This can happen hundreds of times over the span of about four decades. Sometimes the genes in DNA malfunction due to exposure to chemicals or radiation, sometimes these malfunctions in genes are caused by a trait that was inherited from a parent. Cancer occurs when these "mutations" as they are called occur in the genes that are in charge of creating new cells. The mutation is then recreated over and over causing what we know of as cancer. In the case of estrogen, these genes that are in charge of creating new cells are stimulated by estrogen in the target tissue areas (most commonly: the breast tissue) and they continue to produce cells although now they are the mutant cells. Sometimes mutations can occur when the DNA (whose job is to insert information in to developing cells) inserts slightly flawed information into the cells. Estrogen increases the chances of these mutations occurring, often times the mutation results in the production of more mutated cells thus creating cancerous colonies in the tissues of the body. It is estrogen's job (one of many) to stimulate the production. 
One of the many jobs estrogen has is to promote the multiplication of female cells in the target tissues. As discussed before, this can be harmful when a mutation in the target tissues occur. For this reason, scientists have created a drug called Antiestrogens. Antiestrogens attach themselves to estrogen receptors making it impossible for them to do their designed job. The theory is that this will decrease the multiplication of mutated cells in the target tissues that eventually leads to cancer. 
Another estrogen manipulator is what is called "selective estrogen receptor modulators" or "SERMs." SERMs work in a different way than antiestrogens because SERMs are able to tell the receptors to do a variety of things, whereas the antiestrogens' job was simply to stop them from working altogether. SERMs can inhibit one receptor and activate another. This allows the cells to continue working as they would in the uterus, but stop completely in the breast tissues.
Tamoxifen is one such SERM that inhibits cell development in the breast tissue. It does this by attaching itself to the receptor in a way that disallows it to react with co-activators. This is extremely helpful when trying to prevent the spread of cancer in the breast tissues, which is what Tamoxifen does. It cuts off production in the breast tissue while mimicking the characteristics of estrogen in the uterus. It will produce new estrogen cells in the uterus at a normal rate. Though Tamoxifen does not exterminate large these colonies of cancerous cells, it does help with killing off any extra cancer cells after a surgery is performed to remove the major sites of cancer cell production.
As with any other mutant cell, cancer is not a stable molecule. Tamoxifen only works with receptor-positive cancers, which means the cancer growth is stimulated by estrogen. There are receptor-negative cancer cells which cannot be treated by Tamoxifen because it is not stimulated by estrogen, remember that Tamoxifen works by binding to the estrogen receptors of mutated cells. It is also dangerous because of its effects on the uterus. It will continue to act as estrogen does in the uterus, it will stimulate the production of new cells just as estrogen does. This causes an increased chance of cancer in the uterus because of the chance that a cell could become mutated and be reproduced by the prompting of the Tamoxifen. 
Scientists have found xenoestrogen is being used as an additive in many foods. It has been used to preserve oils from spoiling and keeping coloration in shellfish. Xenoestrogen has shown to increase the risk of breast cancer in women and reduce sperm count in men. Scientists are now using a new method of uncovering xenoestrogen (and xenoestrogen-like materials) in foods. [
Adults who consume milk may actually have a higher risk of cancer and cancer-related diseases. Estrogens in milk are known to increase the growth of cancer cells. Recently, scientists have been looking for links between milk and cancer. They have found the presence of some 2-hydroxyestrone estrogens, which are very dangerous. While searching for hormones in milk, scientists found the least in skim milk, though they found the most 2-hydroxyestrones in it as well. Our bodies produce the exact hormones that we need to maintain healthy bodies. The presence of hormones in milk add what's called "noise" to the working hormones already present in our bodies. Scientists have found that estrogens, some of which are found in the milk that was tested, produce tumors in the reproductive tissues of adults. Dermatologist F. William Danby, who teaches at Dartmouth Medical School says "Extra receptors permit more estrogen—including any from milk—to unlock the cellular machinery that can turn tumor growth on." Finally he says, "And this is probably the most important thing(Milk-derived hormones) are being poured into a system that didn’t anticipate them (—at least in adulthood)." 
Compounded estrogen mixtures are popular now for reducing the impact of the aging process. Unfortunately, there are few compound-certified pharmacists in America. If these compounded estrogen (and other hormone) mixtures are mismeasured, they can lead to serious medical problems, and in some cases: death. 
- Health Topic: Estrogen National Women's Health Resource Center, 2/20/02.
- Estrogen Wa.Edu, 6/20/08.
- Estrogen-Like Additives Alessio Amadasi, ScienceDaily, March 5, 2009.
- Milk's Link to Cancer author, publisher, date.
- Estrogen-Induced Proliferation of Existing Mutant Cells National Cancer Institute, 01/28/2005.
- What is estrogen? Karl Harrison, April, 2006.
- What are estrogens? National Cancer Institute, 01/28/2005.
- Shannon, Marilyn; Wile, Dr. Jay L. The Human Body Fearfully and Wonderfully Made! Anderson: Apologia Educational Ministries, Inc., 2001.