Among all breast cancer molecular subtypes, luminal breast cancers (defined by ER and/or PR-positive and HER2-negative) enjoy the best survival rate and are the only subtype which may benefit from long-term endocrine therapy and may be spared from cytotoxic chemotherapy if residual risk following curative surgery is low enough and endocrine therapy alone can counteract the risk of recurrence [4,5]. The gene discussed is ESR1; the disease is breast cancer.