Based on our previous findings showing the dependence of ERα in tumor regression and the involvement of extranuclear ERα in colony inhibition, in this study we determined the subcellular localization of ERα in T47D:A18/PKCα tumors during regression (E2 and RAL) and during proliferation (absence or presence of TAM) using immunofluorescence (IF) confocal microscopy. The gene discussed is ESR1; the disease is neoplasm.