Shah, of New York University School of Medicine, reported that their meta-analysis of more than 10 observational studies suggests that treatment with estrogen alone may reduce the risk of death from breast cancer. Shah said the combination of estrogen and progestin is known to cause breast cancer, but it is not known whether estrogen alone causes breast cancer. Their meta-analysis of different studies showed that patients with breast cancer who were treated with estrogen alone had a significantly lower death rate. This may be because women who used estrogen had more frequent mammograms than those who did not, so that breast cancer could be detected early. The researchers found that the breast cancer incidence ratio (OR) for the estrogen replacement therapy and breast cancer study meta-analysis (52,972 cases) was 1.08; the OR for the seven estrogen-progestin combination therapy and breast cancer study meta-analysis (36,431 cases) was 1.32; and the OR for the nine breast cancer death rates and estrogen replacement therapy meta-analysis (520,620 cases) was 0.81. The researchers concluded that that their findings differ from those of other recent studies in that the increased risk of breast cancer is limited to those currently applying combined estrogen-progestin replacement therapy and not to those using estrogen alone.