Her medical history revealed that, in year X, the patient was diagnosed with right breast cancer and underwent a total right mastectomy and sentinel lymph node biopsy, which revealed invasive lobular carcinoma, estrogen receptor (ER)-positive (> 90%), progesterone receptor (PgR)- positive (> 90%), human epidermal growth factor receptor 2 (HER-2)-negative, and Ki-67 index < 5%. Here, ESR1 is linked to invasive lobular breast carcinoma.