ESR1 and breast cancer: Previous work in our laboratory has demonstrated the expression of IGF-1R adapter proteins, specifically IRS-1, is correlated with poor prognosis in ER positive breast cancer patients.25 Both the Endogenous Hormones and Breast Cancer Collaborative Group26 and European Prospective Investigation into Cancer and Nutrition cohort27 have emphasized the cross talk between ER and IGF-1 receptors increases breast cancer risk.