For breast cancer, the differences of CDC25C (P = 0.001), CDK1 (P = 0.000), CHK1 (P = 0.001), CHK2 (P = 0.000), PLK1 (P = 0.000) and Aurora A (P = 0.000) expression between primary cancer with and without metastasis also had statistical differences (Tables S11–S16). Here, PLK1 is linked to cancer.