Breast cancer (BC) is a heterogeneous disease, comprising multiple subgroups of varying molecular signatures, prognoses, and responses to therapies.1 From the clinical perspective, BC can be subdivided into three major subtypes: tumors expressing estrogen receptor (ER) and/or progesterone receptor (PgR; commonly referred to as hormone receptor‐positive [HR+]), ErbB2‐amplified (also known as human epidermal receptor 2‐amplified [HER2+]), and triple‐negative BC (TNBC) due to the absence of ER/PgR and normal or negative HER2 expression.2 This evidence concerns the gene ESR1 and breast cancer.