CDK2 and neoplasm: The expression of cyclin E1 (CCNE1) and cyclin E2 (CCNE2) was found to be strongly activated by the E2F transcription factors, such as E2F1 [41, 42], and the CDK2-cyclin E complex phosphorylated and inactivated Rb, while the phosphorylated Rb released E2F transcription factors, thereby promoting cell-cycle progression from G1 to S phase led to tumor regressions [43–45].