ESR1 and breast cancer: The degree to which this observation in TransATAC may be generalizable is not known, but it is supported by the data related to the switching of the importance of estrogen signaling at 5 years being very similar to that reported by Bianchini et al.25 A meta-analysis from Early Breast Cancer Trialist Collaborative Group data of 74,194 women with primary ER-positive breast cancer who were scheduled to have received 5 years of endocrine therapy also found that higher progesterone receptor expression reflected better outcomes only in the first 5 years after surgery and not beyond 5 years.26