The Oncotype DX® Breast Cancer assay (Genomic Health, Inc., Redwood City, CA) has been validated as a prognosticator and a predictor of the likelihood of chemotherapy benefit in estrogen receptor-positive (ER+) early invasive breast cancer (Paik et al. 2004; Paik et al. 2006; Habel et al. 2006; Goldstein et al. 2008; Albain et al. 2010; Dowsett et al. 2010; Toi et al. 2010; Mamounas et al. 2012). Here, ESR1 is linked to breast cancer.