Worldwide, breast cancer is the most common type of cancer and the most common cause of cancer-related mortality among women. In the United States, approximately 212,920 new cases of invasive breast cancer, 61,980 in situ cases, and 40,970 deaths were expected to have occurred in 2006. In women, breast cancer accounts for 26% of new cases of cancer and 15% of cancer deaths, second only to lung cancer as a cause of cancer-specific death. Approximately 1% of breast cancers occur in males and 90% are estrogen receptor (ER)-positive. Incidence rates continued to increase until 2002, likely reflecting increase use of mamographic screening, but recently have been reported to be declining. Part of that decline may be due to a decrease in the use of postmenopausal hormone replacement therapy. Although incidence rates (all races combined) are substantially higher for women age 50 and older (375.0 per 100,000) compared with women younger than 50 years (42.5 per 100,000), approximately 23% of breast cancers are diagnosed in women younger than 50 years, because those women represent 73% of the female population.