Commonly, PLK1 and PLK4 have been considered as oncogenes whereas tumor-suppressive functions are attributed to PLK2, PLK3, and PLK5. With increasing knowledge, it becomes evident, however, that the participation and the mechanisms of action of the polo-like kinase family members in carcinogenesis are much more diverse and complex and far away from being fully understood. Here, PLK4 is linked to neoplasm.