CDK1 and breast cancer: Given that LEM4 overexpression enabled MCF7 cells to be estrogen-independent for growth, and the expression of CDK1, cyclin D1, CDK4/6, and CDK2, cyclin E were up-regulated in both MCF7-LEM4 and T47D-LEM4 cells, we sought to determine whether the LEM4 overexpression could account for tamoxifen resistance in ER+ breast cancers.