In breast cancer, the pineal hormone controls the transcriptional activity of the estradiol-estrogen receptor complex, therefore behaving as a selective estrogen receptor modulator (SERM), and also diminishes the production of estrogens through the regulation of the enzymes involved in estrogen activation/inactivation, thus behaving as a selective estrogen enzyme modulator (SEEM) [9,10,11]. This evidence concerns the gene ESR1 and breast cancer.