ESR1 and neoplasm: We update progress in our understanding of the stromal cells populating the microenvironments of diverse primary and metastatic ER+ tumors, the distinct estrogen receptors that mediate effects of the range of estrogenic ligands and modifiers of estrogen synthesis to which these cells may be exposed over the course of disease development and treatment, and the effects of estrogen activity on local communication networks to modulate immune activity and properties of the extracellular matrix in the ER+ tumor environment.