ESR1 and breast cancer: The results (Figure 1c and 1d) indicated first that tamoxifen did not up-regulate the expression of p27 in both MDA-MB-231 and MCF7 cells, but other four anti-cancer agents up-regulated the expression of p27 in both ER-positive (also LKB1-positive) and ER-negative (also LKB1-negative) human breast cancer cells in vitro. Next, expression of p27 protein in ER-negative MDA-MB-231 cells was examined by Western immunoblot analysis.