PLK1 and cancer: The proliferation modules also contain genes frequently described as markers of proliferation in different types of cancer [45]: PLK1, BUB1, CCNA2, CCNB1, CCNB2, CCNE2, FOXM1, and TOP2A. These genes are derived from the functional intersection of the enlarged gene expression signatures, indicating that proliferation is a major driver of the prognosis gene signatures.