PLK1 and breast cancer: Ali et al. compared the prognostic value of proliferative markers, such as MCM2, Ki67, Aurora A, polo-like kinase 1 (PLK1), GMNN and phosphorylated-histone H3 (PHH3), based on their differential expression in different phases of the cell cycle, then showed that Aurora A is the best prognostic factor, outperforming Ki67 in ER positive breast cancer [15].