Estrogen receptor (ER) status is used in classifying breast cancer (Heng et al. 2017; Søkilde et al. 2019) as well as for prognosis and treatment eg (Gu et al. 2020; López-Sánchez et al. 2020; Reis-Filho and Pusztai 2011). The gene discussed is ESR1; the disease is breast cancer.