Mammograms are special x-rays of the breast that can identify breast cancers before a woman or her doctor can feel a lump in the breast. However, mammograms are not totally accurate. They sometimes suggest a cancer when it is not present (a false-positive result). Other times, mammograms look normal even though a cancer is present (a false-negative result). Younger women's breasts contain less fat and are denser than older women's breasts. In addition to age, breast density is affected by hormones, such as combinations of progestin and estrogen or estrogen alone. When a woman takes these hormones, her breasts become less fatty and denser. Mammograms of dense breasts are harder to read than mammograms of fatty breasts. However, the way that breast density, age, and hormone therapy influence the accuracy of mammograms (alone and in combination) is not well understood.