Another model proposes that cyclin D levels remain nearly constant through G1.[11] Rb is mono-phosphorylated during early to mid-G1by cyclin D-Cdk4,6, opposing the idea that its activity gradually increases. Cyclin D dependent monophosphorylated Rb still interacts with E2F transcription factors in a way that inhibits transcription of enzymes that drive the G1/S transition. Rather, E2F dependent transcription activity increases when that of Cdk2 increases and hyperphosphorylates Rb towards the end of G1.[12] Rb may not be the only target for cyclin D to promote cell proliferation and progression through the cell cycle.