ESR1 and breast cancer: However, several studies have shown that, when the prognostic assays are compared to the gene expression based sub-classification of breast cancers, these assays are essentially identifying Luminal A tumors (low grade, highly ER+ breast cancers, HER2-) as being good prognosis, and Luminal B, ER+ breast cancers (which are ER+, mostly intermediate-to-high grade, some with HER2 amplification) as poor prognosis [12]–[14].