The effects of melatonin on the inhibition of estrogen-mediated proliferation of human ER-α positive BC cell lines (e.g., MCF-7) are well known [42]; however, despite several studies showing an anti-proliferative effect of melatonin on the triple-negative MDA-MB-231 cell line [43,44], others have reported that melatonin did not significantly inhibit in vitro proliferation of MDA-MB-231 cells [45,46]. This evidence concerns the gene ESR1 and breast cancer.