ESR1 and cancer: This was also reflected in the subset of patients with quantitative ER and PR testing results, which were higher in ERBB2-low (mean [SD], 82.5% [31.1%] ER expression, 56.4% [39.7%] PR expression) compared with ERBB2-negative (mean [SD], 70.8% [40.2%] ER expression, 49.8% [42.2%] PR expression) cancers.