ESR1 and invasive ductal breast carcinoma: The patient is a 61-year-old woman with a diagnosis of left breast infiltrating ductal carcinoma, pT1cN0, grade G2, ER 98%, PGR 20%, HER2+, who underwent quadrantectomy and axillary lymph node dissection followed by adjuvant chemotherapy, radiotherapy (50 Gy in 25 fractions + 10 Gy in 5 fractions by photons), and letrozole administration for 5 years.