ESR1 and breast carcinoma: Furthermore, analysis of GREB1 mRNA in 225 patients from the Tamoxifen (TAM) arm of 89-30-52 indicates that GREB1 can serve as an independent predictor of good disease-free survival in Tamoxifen-treated patients in ways even more reliable than ER or PgR, which itself has historical precedence as a clinical prognosticator in breast cancer patients in response to endocrine therapy (Table 1) [8], [51], [52].