sentences:  Specifically, E2F helps to activate Cyclin E and Cyclin A, which are constituents of other Cdk/Cyclin complexes and are involved in the DNA replication process and other downstream mitotic processes.There are multiple regulation points within this signaling pathway.First and foremost, under non-dividing conditions multiple proteins can inhibit the Cyclin D/Cdk4 complex by binding Cdk4 and inhibiting its association with Cyclin D. Primarily, this is accomplished by p27 but it can also be done by p16 and p21.