Although in eight women in the United States will most likely develop breast cancer according to commonly used statistics, recent reports indicate that breast cancer rates are on the decline, mostly because of improved recognition, prevention, and treatment. But there are some crucial steps in the process of beast cancer evaluation, being the most important the need to test the tumor tissue removed during a biopsy or surgery to determine if it has estrogen and progesterone receptors – molecules that the hormones bind to. But what are estrogen and progesterone receptors and how do they affect cancer cells? Cancerous cells may have none, one, or both receptors. Breast cancers that have estrogen receptors are called ER-positive (or ER+). Those with progesterone receptors are referred to as PR-positive (or PR+). Also, in addition to hormone receptors, some breast cancers have high levels of a growth-promoting protein called HER2/neu. If a tumor has this property, it is called HER2-positive. HER2 positive cancers are more aggressive than HER2 negative cancer.