ESR1 and breast cancer: BC cells across all subtypes (ER+, HER2+, TNBC) secrete various factors such as TGF-β, IL-10, CCL2 & C-C motif chemokine ligand 5 (CCL5; encoded by CCL5), colony-stimulating factors, prostaglandin E2, and indoleamine 2,3-Dioxygenase, that prevent anti-tumor immunity and create an immunosuppressive environment [161–165].