PLK1 and cancer: Although PLK1 is the focus of our study for its significant growth inhibition on breast cancer, availability of small-molecular inhibitors, and the safety data in clinical trials of different cancer treatment [28,40,49], several other active kinases identified in this study deserve further study for their roles in TICs in breast cancer, such as SKP2 and PLAU (uPA), which inhibited the growth of sorted CD44high/CD24-/low cells of SUM149.