The effects of 4-hydroxytamoxifen (4-OH-tamoxifen) and tamoxifen on the expression of p27 in human breast cancer cells in vitro were investigated using p27-luciferase reporter plasmids containing the following proximal 5'-upstream regions of p27 gene, namely -1797 p27 (p27-Kpn I), -774 p27 (p27-Apa I), and -575 p27 (p27-5'UTR) (Figure 1a). The gene discussed is CDKN1B; the disease is breast cancer.