Both clinical trials described in this manuscript were approved by the local institutional review boards that enrolled patients into the studies (3 (link),5 (link)). The P024 protocol compared 4 months of neoadjuvant letrozole therapy with 4 months of neoadjuvant tamoxifen therapy in postmenopausal women with clinical stage 2 and 3 hormone receptor–positive breast cancers (classified as at least 10% nuclear staining for estrogen receptor [ER] and/or progesterone receptor [PgR]) who were ineligible for breast conservative surgery (3 (link)). The clinical findings, tumor bank characteristics, and biomarker measurements have been described previously (3 (link),4 (link),7 (link)). The cut point for ER positivity for central laboratory analysis was an Allred score of 3 (8 (link)). Information on tumor grade, clinical response by caliper measurements, definitive pathological staging at surgery, and chemotherapy administration was collected prospectively. Patients in P024 were recommended to receive adjuvant tamoxifen for 5 years. The IMPACT study design, short- and long-term outcomes, and biomarker methodology have also been described previously (5 (link),6 (link),9 (link)). For the validation analysis, we compiled information on surgical stage, surgical specimen Ki67 proliferation antigen levels, ER data, duration of follow-up, and relapse dates. The IMPACT study used the H-score (10 (link)) to assess ER status. We converted the H-score ER cutoff to an Allred score ER cutoff for the analyses. An Allred score of 2 can be derived in only one way, ie, less than 1% of cells staining weakly, which equates to an H-score of less than 1. Thus, it is valid to use an H-score of at least 1 as the equivalent of an Allred score of at least 3 as the threshold for ER positivity.