ESR1 and breast cancer: In summary, these results suggested that 4-hydroxytamoxifen (but not tamoxifen), dexamethasone, 9-cis-retinoic acid (9cRA) and probably all-trans-retinoic acid (atRA) up-regulated the expression of p27 in both ER-positive and negative human breast cancer cells in vitro (Figures 1c, 1d and 1e).