Artonin E from the bark of A. elasticus showed strong cytotoxic activity against human estrogen receptor (ER+)-positive (MCF-7) and human estrogen receptor (ER−)-negative breast cancer cells (MDA-MB 231), with IC50 values of 2.6 ± 0.3 μg mL−1 and 13.5 ± 1.3 μg mL−1, respectively. Here, ESR1 is linked to breast cancer.