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The Early Breast Cancer Trialists’ Collaborative Group, an international team of investigators based at the University of Oxford, pooled data from over 60,000 women in 88 clinical trials who had been diagnosed with estrogen receptor positive breast cancer between 1976 and 2011, and prescribed anti-estrogen therapy, such as tamoxifen or an aromatase inhibitor, for 5 years. These women who had no recurrence in the first 5 years, then stopped treatment and their progress was followed for up to 15 further years.