ESR1 and breast cancer: This plasmid was transfected into the estrogen receptor (ER) - positive as well as negative human breast cancer cells in vitro and then the transfected cells were exposed to 1 μM each of the following five different anti-cancer agents, namely tamoxifen, 4-hydroxytamoxifen, dexamethasone, all-trans -retinoic acid (atRA), and 9-cis-retinoic acid (9cRA) for 24 hours.