Current guidelines state that patients with an estrogen receptor-positive (ER+)/human epidermal growth factor receptor 2-negative (HER2-) (Luminal) tumour subtype should be offered endocrine therapy, and if possible in combination with a cyclin-dependent kinase (CDK)4/6-inhibitor as first- (preferably) or second-line treatment, except for patients with visceral crises when chemotherapy is recommended [6]. The gene discussed is ESR1; the disease is neoplasm.