In a study published in 2011 in the Asian Pacific Journal of Cancer Prevention, lemons were shown to induce apoptosis or cell death of breast cancer tumor cells. The cancer cells studied were MCF-7 cells. In the study, genes P53, caspase 3, and Bax were activated by the lemon extract, inducing apoptosis, or cancer cell death of the breast cancer cells. (i) The P53 gene is known to prevent cancer by acting as a tumor suppressor. In another study published in 2013, compounds found in lemons called limonoids were studied to see if they could be cytotoxic to human breast cancer cells. The bioactive phytochemicals present in the citrus were extracted from powdered lemon seeds and isolated, and 9 purified limonoids and 4 modified limonoid compounds were studied. Of the 13 compounds, 11 were found to be cytotoxic to MCF-7 and 8 cytotoxic to MDA-MB-231 cancer cells. MCF-7 are estrogen dependent cells and MDA-MB-231 are not. While the compounds did suppress production of aromatase, an enzyme responsible for the production of estrogen, compounds found in lemons were shown to induce inhibition of proliferation of the cancer cells by activation of the caspsase-7 gene. (ii) Some of the compounds in this study were also found to have antioxidant properties in a separate study published in the Journal of Agricultural and Food Chemistry, in 2005 (iii). Free radicals cause cancer by bouncing around and damaging DNA. Antioxidants prevent this from happening by binding to and therefore removing free radicals.