Schematic representation of experimental setup. One hundred and twenty female, nude mice were ovariectomized at 10 weeks of age. After a week MCF7-ARO cells were injected into the flanks (~3 × 106 cells/site) and the animals were separated into 8 groups (n = 15). All the animals received 0.1 mg DHEA as daily subcutaneous injection. Ovariectomized control group animals did not receive any additional treatment, AI group animals received 60 μg anastrozole as subcutaneous injection for 7 weeks, E + P (40/6) group animals were implanted with E and P pellets that results in 40 pg E and 6 ng of P in circulation, E + P (100/6) delivers E and P to achieve 100 pg E and 6 ng P in circulation, E + P + T group animals received T pellets along with E and P pellets to achieve serum concentration of E-100 pg, P-6 ng and T-50 ng. AI + E + P group animals received 60 μg anastrozole as subcutaneous injection, for 7 weeks and E and P pellets to release E-100 pg and P-6 ng in circulation. Estrous group animals received E pellets releasing 25 pg of E in circulation and additionally they received 0.1 μg of E at day 1 and 0.5 mg of P at day 3 as a subcutaneous injection, to mimic estrus cycle. The estrus cycle treatment was repeated every 5 days, till the end of the experiment. Menstrual group animals were implanted with E pellets releasing 100 pg of E in circulation and 1 mg of P as a subcutaneous injection at day 14 through day 28 and the procedure was repeated every month to mimic human menstrual cycle. A set of 3 mice per group was terminated at 5, 10 and 15-month time points and remaining 6 animals were observed until ~30 months. A survival surgery was performed to remove tumors if they reach 500 mm3 in size.