This happens in response to sensors of external growth-regulatory signals and cell growth, and Rb is phosphorylated as a result. Rb reduces its binding to E2F and thereby allows E2F-mediated activation of the transcription of cyclin E and cyclin A, which bind to Cdk1 and Cdk2 respectively to create complexes that continue with Rb phosphorylation. Cyclin A and E dependent kinase complexes also function to inhibit the E3 ubiquitin ligase APC/C activating subunit Cdh1 through phosphorylation, which stabilizes substrates such as cyclin A. The coordinated activation of this sequence of interrelated positive feedback loops through cyclins and cyclin dependent kinases drives commitment to cell division to and past the G1/S checkpoint. Another model proposes that cyclin D levels remain nearly constant through G1. 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. Rb may not be the only target for cyclin D to promote cell proliferation and progression through the cell cycle. The cyclin D-Cdk4,6, complex, through phosphorylation and inactivation of metabolic enzymes, also influences cell survival. Through close analysis of different Rb-docking helices, a consensus helix sequence motif was identified, which can be utilized to identify potential non-canonical substrates that cyclin D-Cdk4,6 could use to promote proliferation. RxL- and LxCxE- based docking mutations broadly affect cyclin-Cdk complexes. Mutations of key Rb residues previously observed to be needed for Cdk complex docking interactions result in reduced overall kinase activity towards Rb. The LxCxE binding cleft in the Rb pocket domain, which has been shown to interact with proteins such as cyclin D and viral oncoproteins, has only a marginal 1.7 fold reduction in phosphorylation by cyclin D-Cdk4,6 when removed. Similarly, when the RxL motif, shown to interact with the S phase cyclins E and A, is removed, cyclin D-Cdk4,6 activity has a 4.1 fold reduction.