Our objectives were to determine the antioxidant and antiproliferative activity of the methanolic extract of the following cocoa plant parts: leaf, bark, husk, fermented and unfermented shell, pith, root, and cherelle, against the estrogen receptor–positive (MCF-7) and estrogen receptor–negative (MDA-MB-231) breast cancer cell lines; liver (HepG2), colon (HT-29), lung (A549), and cervical (HeLa) cancer cell lines; and a normal liver cell line (WRL-68). This evidence concerns the gene ESR1 and breast cancer.