ESR1 and breast cancer: Then, we investigated the potential cytotoxic properties via oxidative stress of the natural product 7-hydroxy-3,4-dihydrocadalene and the semi-synthetic cadinanes derivatives 7-(phenylcarbamate)-3,4-dihydrocadalene and 7-(phenylcarbamate)-cadalene against breast cancer cells using (Z)-4-hydroxytamoxifen as a positive cytotoxic control since is a Selective Estrogen Receptor modulator (SERM) that has remained the antihormonal therapy of choice for the treatment of Estrogen Receptor (ER) positive breast cancer.