ESR1 and neoplasm: Our findings on MPA treatment of BT474 cells are consistent with a model of Tam resistance in the clinic where, firstly, exposure to an endogenous progestational milieu, mimicked in our study with MPA, of ER+/PR + tumors displaying at diagnosis p-c-Jun nuclear presence but lacking p-c-Jun/PR colocalization, will enhance p-c-Jun levels and induce the assembly of an AP-1/PR complex and tumor proliferation.