CDKN1B and breast cancer: 4-Hydroxytamoxifen (but not tamoxifen) up-regulated expression of p27 in estrogen receptor (ER) -positive as well as negative breast cancer cells in vitro [also see the reference [10], suggesting that 4-hydroxytamoxifen up-regulates the expression of p27 regardless of the status of estrogen receptor in the breast cancer cells.