Abstract Deregulation of the cell division cycle is critical for cancer development. Transition from G1 to S phase of the cell cycle is promoted by D-type cyclins (D1, D2 and D3). Cyclin D1 is a critical regulator of cell cycle progression due to its ability to activate cyclin dependent kinases (CDK) 4 and 6, which phosphorylate and inactivate the retinoblastoma protein (pRb) and activates the transcription factor E2F1 to promote the expression of genes essential for G1/S progression. Importantly, mutations in the tumor suppressor gene RB1 or inactivation of the RB tumor suppressor pathway occurs in almost all human malignancies and one such mechanism involves cyclin D1 overexpression. Even in cancer cells with amplified or elevated cyclin D1 mRNA, the Cyclin D1 protein rapidly declines as cells progress through the G1 phase of the cell cycle to reach undetectable levels in S-phase, and this is thought to be dependent on the Skp1-Cul1-F-box protein (SCF) E3 ubiquitin ligase complex. The identity of the E3 ubiquitin ligase that is specifically involved in downregulating cyclin D1 during S-phase of the cell cycle or the functional significance of cyclin D1 downregulation however, remains unknown, as recent genetic studies in mice demonstrated that the steady state levels or stability of cyclin D1 protein, are not altered following the inactivation of the known regulators of cyclin D1 stability. Using a genetic si-RNA-based screen, we have identified a novel E3 ubiquitin ligase; SCF-FBXW5, which specifically interacts with and downregulates cyclin D1 protein, both in unperturbed proliferating cancer cells and following DNA damage. Importantly, inactivation of this E3 ubiquitin ligase inhibits the proliferation of cancer cells primarily through cyclin D1-dependent inhibition of S-phase progression and sensitizes cancer cells to DNA damage induced by UV or ionizing radiation. These findings identify a novel axis of regulating cyclin D1 stability in cancer cells with implications for targeting this axis to inhibit cyclin D1-driven cancers. Citation Format: Fadila Guessous, Jinho Heo, Naga Vaddadi, Tarek Abbas. Novel regulation of cyclin D1 stability and the DNA damage response. [abstract]. In: Proceedings of the 106th Annual Meeting of the American Association for Cancer Research; 2015 Apr 18-22; Philadelphia, PA. Philadelphia (PA): AACR; Cancer Res 2015;75(15 Suppl):Abstract nr 3786. doi:10.1158/1538-7445.AM2015-3786