We then exposed six widely used breast cancer cell lines (a subset of the lines described below) to nine-concentration of each of four drugs having different mechanisms of action: CDK1/2 inhibitor BMS-265246, CDK4/6 inhibitor palbociclib, microtubule stabilizing drug paclitaxel, and PLK1 kinase inhibitor volasertib. This evidence concerns the gene PLK1 and breast cancer.