CDKN1B and breast cancer: A modest increase in p21WAF1/CIP1 and p27Kip1 expression was observed after treatment of E2-stimulated MCF-7 cells with ICI 182 780, thus corroborating the role of these CDKIs as critical mediators of the growth inhibitory effects of antiestrogens in breast cancer cells.43, 44 Remarkably, p21WAF1/CIP1 and p27Kip1 expression was substantially increased in E2-stimulated MCF-7 cells treated with increasing concentrations of C75 (Figure 6a).