Background: CDK4/6 inhibitors (CDK4/6i) such as palbociclib and ribociclib are used to treat ER+/HER2- breast cancer, but patients can develop resistance via many mechanisms, several of which converge on the upregulation of CDK6. This has been shown to limit the effectiveness of CDK4/6i in ER+ breast cancer with up to 20% patients exhibiting innate resistance and up to 70% patients developing acquired resistance after 3 years on therapy (Scheidemann, 2021, doi:10.3390/ijms222212292). Methods: To address this limitation, we utilized our PRODEGY platform of Cereblon (CRBN) binders to synthesize CRBN mediated CDK4/6 bifunctional degraders to potently inhibit tumor growth for treatment of naïve ER+/HER2- breast cancer and CDK4/6i resistant tumors. Results: Target degradation by immunoblot analysis of the triple negative breast cancer (TNBC) cell line, MDA-MB-231, treated with our CDK4/6 bifunctional degraders for 6 hours showed up to 85% degradation of CDK4 and CDK6 with DC₅₀s of 1-100nM. CDK4/6 phosphorylates the protein RB which releases the transcription factor E2F, inducing the expression of genes which promote cell cycle progression. Analysis of RB phosphorylation by in-cell western upon 24 hours of CDK4/6 degrader treatment showed phospho-RB IC₅₀s at <30nM. Cell cycle analysis by staining with propidium iodide after 24 hours of treatment with CDK4/6 degraders induced G0/G1 cell cycle arrest at concentrations as low as 10nM. We used a 2D colony formation assay (CFA) as a readout for inhibition of proliferation by cell cycle arrest. Our CDK4/6 degraders showed potent inhibition of cell proliferation with CFA IC₅₀s of <100nM in TNBC cell lines and <25nM in ER+ cell lines, including MCF7, T47D and ZR751 compared to CDK4/6i which ranged from 200nM to 500nM in MCF7 cells. We demonstrated that our CDK4/6 bifunctional degraders were significantly more potent in vitro than the CDK4/6i ribociclib and palbociclib, and the increased activity was due to CRBN mediated target degradation. Our CDK4/6 bifunctional degraders display excellent pharmacokinetic properties in mice with half-lives between 2-10 hours, oral bioavailability between 50-96%. MCF7 xenograft results with our proof-of-concept CDK4/6 degrader showed dose-dependent tumor growth inhibition and greater potency compared to the clinical CDK4/6i ribociclib. We saw tumor regression with our degrader at higher doses which we did not see at any dose of CDK4/6i. Conclusions: Our CDK4/6 bifunctional degraders display excellent single agent activity in vitro and in vivo particularly in comparison to clinically approved CDK4/6i, indicating that using a degrader approach to targeting this pathway may be more effective than current inhibitor therapies.