ESR1 and breast cancer: Clinical samples from breast cancer patients, show upregulated expression of PD-L1 and is associated with poor prognostic features: ductal type, large tumor size, high grade, ER−, PR−, ERB-B2 receptor tyrosine kinase 2 (ERBB2)+, high proliferation rate and aggressive molecular subtypes (basal and ERBB2-enriched) [138].