Deletion analysis indicates that 4-hydroxytamoxifen, dexamethasone, all-trans-retinoic acid (atRA) and 9-cis-retinoic acid (9cRA) activate the proximal 5'-upstream region (-1797) of p27 gene through its 5'-untranslated region (5'-UTR) (-575). (a) The 5'-deletion mutants of -1797 p27 (p27-Kip I) used in this experiment were -774 p27 (p27-Apa I) and -575 p27 (p27-5'UTR). Two additional deletion mutants, -435 p27 (p27-MB) and -417 p27 (p27-IRES), were also used in this experiments, but the data are not shown. The estrogen receptor (ER)-negative MDA-MB-231 human breast cancer cells were transfected with these deletion mutants and then exposed to 1 μM each of (b) tamoxifen, (c) 4-hydroxytamoxifen (4-OH-tamoxifen), (d) dexamethasone, (e) all-trans-retinoic acid (atRA) and (f) 9-cis-retinoic acid (9cRA) for 24 hours. All assays were performed in triplicates and the transfection experiments were repeated three times.