Final answer:
Cyclins activate G2-Cdk and G1/S-Cdks in regard to inhibition with Rb proteins.
Step-by-step explanation:
The activation of G2-Cdk and G1/S-Cdks is primarily regulated by cyclins. Cyclins are proteins that bind to cyclin-dependent kinases (Cdks) and activate them, allowing them to phosphorylate target proteins and drive the cell cycle forward. Inhibition of Rb proteins is relieved when phosphorylation of Rb occurs, allowing the release of transcription factors such as E2F that promote the production of proteins necessary for the G1/S transition.