When comparing SLNB versus ALND in patients with ER+ HER2− breast cancer with 3–5 positive lymph nodes, we found that patients in the SLNB group were more likely to be > 50 years, have 3 positive lymph nodes, lower CCI, grade 1–2 disease, no LVI, and lobular histology. The gene discussed is ESR1; the disease is breast carcinoma.