Breast cancer remains the most common cause of cancer among women in the United States, and it results in more deaths from cancer among women than any other type of cancer, except lung cancer. Over 40,000 women die from breast cancer in the United States each year. A long history of research, now coupled with the new information emerging from the field of molecular genetics, is beginning to explain the basic steps leading to breast cancer, and it will enable the development of novel treatment and prevention strategies.

Almost all breast cancers begin in the glandular structures in the breast that, during lactation, produce milk. These mammary glands are under the control of reproductive hormones that stimulate the monthly cycle of gland expansion and shrinkage, which is a feature of the regular menstrual cycle. Many of the factors associated with the development of breast cancer appear to...