In this well-characterized case series of patients with ER-positive/HER2-negative ILC, a small subgroup of 14 patients (7%) was found to have high expression of the estrogen receptor coactivator AIB1, and five of them died from breast cancer within approximately 5 years, translating to a high cumulative 5-year mortality in this subgroup compared to that in the large subgroup of patients with lower, or no, expression of AIB1 (Fig. 2). Here, ESR1 is linked to breast cancer.