Patients were classified to breast cancer subtypes defined by immunohistochemistry as follows: Luminal A (ER-positive and/or PgR-positive, HER2-negative, Ki67low); Luminal B (ER-positive and/or PgR-positive, HER2-negative, Ki67high); Luminal-HER2 (ER-positive and/or PgR-positive, HER2-positive); HER2-enriched (ER-negative, PgR-negative, HER2-positive); and triple-negative breast cancer (TNBC) (ER-negative, PgR-negative, HER2-negative). This evidence concerns the gene ESR1 and breast cancer.