ESR1 and breast cancer: Using this approach, we first focused exclusively on estrogen‐receptor (ER) positive disease and show that ER + tumors displaying either the LumB expression‐based phenotype or positivity for the Epi‐LumB markers (grouped together as “high‐risk luminal”) have approximately 5‐fold increased risk for breast cancer‐specific death (Hazards ratio = 4.63; P = 0.035) and, importantly, this effect was independent of the LumB expression‐based phenotype alone in a model allowing adjustment for age‐ and year at diagnosis (Figure 6B).