The mechanisms by which melatonin exerts its antitumor actions include antiestrogenic actions such as regulation of ER expression, transactivation, and modulation of enzymes involved in the local synthesis of estrogens, as well as modulation of the cell cycle, stimulation of cell differentiation and apoptosis, suppression of telomerase activity, antioxidant effects, anti-angiogenesis, prevention of circadian disruption, inhibition of metastasis, modulation of epigenetic factors, suppression of tumor metabolism and activation of the immune system [105]. The gene discussed is ESR1; the disease is neoplasm.