These include patients aged over 50 years, diagnosed with small (≤2 cm), unifocal, estrogen-receptor (ER) positive, grade 1 or 2 non-lobular invasive carcinoma or pure DCIS ≤2.5 cm and resection margin of at least 2 mm according to the American Society for Radiation Oncology (ASTRO)recommendations [3]. The gene discussed is ESR1; the disease is ductal breast carcinoma in situ.