Breast cancer is the most frequent cancer in women and the second leading cause of death from cancer in women in the United States. Breast cancer is a type of cancer that develops in the breast tissue, most usually in the inner lining of milk ducts or the lobules that supply milk to the ducts. Breast cancer is the second most frequent type of non-skin cancer (after lung cancer) and the fifth most common cause of cancer mortality in women worldwide, accounting for 10.4% of all cancer cases in women. Breast cancer claimed the lives of 519,000 people globally in 2004. Breast cancer is 100 times more common in women than in men, yet males have a worse prognosis due to detection delays. Cancer cells have DNA and RNA that are very similar (but not identical) to cells from the organism from whence they came. This is why they aren't always identified by the immune system, especially if it's already compromised.
