ESR1 and triple-negative breast carcinoma: However, one-third of patients with aggressive triple-negative breast cancer (TNBC), representing 17–20 percent of all breast cancers [5–7], may relapse more frequently compared to receptor-positive subtypes [i.e., estrogen receptor (ER), progesterone receptor (PR), or human epidermal growth factor receptor 2 (HER-2)].