Based on the idea that breast cancer treatment is particularly complicated by cancer stem cells, and that the control of the disease requires the inhibition of these cells, the objective of this study was to evaluate the effect of melatonin on regulation of the OCT4/POU5F1 by the ERα in BCSC after induction with tumor initiation chemical BPA and with E2. Here, POU5F1 is linked to breast carcinoma.