Introduction: Our previous work has shown that the loss of Rab25 occurs in estrogen receptor (ER) and progesterone receptor (PR) negative breast cancer cell lines and human tumor samples and seems to correlate with an active ras pathway. We have examined the expression levels of rab25 in breast tissue and have investigated the mechanism by which Rab25 exerts its anti-tumorigenic effect. Loss of Rab25 cooperates with mutant Ras to promote tumorigenesis. HIF-1α levels are reduced in Rab25 expressing cells. We have assessed Rab25 binding to the HIF-1α promoter and localized Rab25 to the nucleus, suggesting a novel mechanism for Rab25's antitumor activity and a novel function for Rab25. Materials and Methods: Cell culture – Normal mammary tissue and the RAO-3 cell line was grown in DFCI-1 medium. Cells were transduced with retroviral supernatant and selected. Expression of Rab25 in transduced cells was determined by RT-PCR and western blot. qPCR analysis- Expression of HIF-1α and Rab25 were quantified using real-time PCR with fluorescence detection. Luciferase report assay – We obtained the HIF-1α promoter fragment and inserted the promoter sequence in the pGL4 luciferase vector and the Luciferase Assay was then performed. Western blot- Protein extracts from both cytosolic and nuclear fraction were tested for Rab25 expression. Immunofluorescence- Frozen mammary tissue slides were stained with luminal and basal markers as well as antibody for Rab25 to confirm localization of Rab25 in both luminal and basal mammary epithelium. Results: 1. High level expression of Rab25 was found in luminal epithelium in normal human breast tissue. Basal epithelium also expressed Rab25. 2. Tumorigenic mammary epithelial cell lines, RAO-3 and RAO-4, have lost Rab 25 expression with nearly 1/10,000 the level of Rab25 of normal mammary basal epithelium. 3. Rab25 localizes to the nucleus and reduces luciferase activity from the HIF-1α promoter. 4. Expression of Rab25 substantially reduces HIF-1α levels. Conclusions: 1. Rab25 is a complex molecule that has been described as an intracellular transport protein. 2. It is also a tumor modulator. It can regulate tumor growth and angiogenesis. 3. Rab25 expression can be detected in both luminal epithelium and basal epithelium. 4. Our data indicate novel functions for Rab25 including reduction in HIF-1α expression and localization to the nucleus. Citation Format: {Authors}. {Abstract title} [abstract]. In: Proceedings of the 102nd Annual Meeting of the American Association for Cancer Research; 2011 Apr 2-6; Orlando, FL. Philadelphia (PA): AACR; Cancer Res 2011;71(8 Suppl):Abstract nr 2111. doi:10.1158/1538-7445.AM2011-2111