ough protecting against cancer by regulating the cell cycle and by increasing apoptosis rather than by direct participation in the repair of DNA damage. DDB2 and Breast Tumor Growth Even if DDB2 is considered as a tumor suppressor, we proposed that this protein could play a role in breast cancer. This hypothesis was based on a previous study showing an interaction IC261 site between DDB2 and the transcription factor E2F1, a proliferative marker in breast carcinoma. Also, the aim of this study was to examine the expression of the DDB2 gene in various breast cancer cell lines and in tumors from patients. The surprising results showing an overexpression of DDB2 in ER-positive but not in ER-negative breast cancers led us to investigate the role of this protein in breast cancer cells. This study provides evidence for the first 
time that DDB2 plays an important role in cell cycle progression of breast cancer cells. Results DDB2 gene is overexpressed in ER-positive breast cancer cell lines The expression of DDB2 was assessed in both the ER-negative and ER-positive breast cancer cells and was compared to that in the normal human mammary epithelial cells, at both the transcriptional and the translational levels. The DDB2 mRNA level estimated by RT-PCR analysis was 9.1-fold higher for MCF-7 and 5.1-fold higher for T47D cell lines than for HMEC cells. No significant difference was observed between the DDB2 mRNA levels of the ER-negative cancer cells and the HMEC cells. These results correlated with the DDB2 protein content. Western blot analysis showed that the MCF-7 and T47D cells had similar and very high levels of DDB2 protein, in contrast to the very low levels in MDAMB231, SKBR3 and HMEC cells. No significant difference was observed between the DDB1 mRNA levels of the cell lines studied. However, the MCF-7 cells had respectively 3.7and 2.5-fold higher levels of DDB1 protein than the SKBR3 and MDA-MB231 cells. No significant difference was observed between the DDB1 protein levels of the MCF-7, T47D and HMEC cell lines. The MCF-7 cells expressing the highest basal DDB2 levels were submitted to in situ immunofluorescence in order to analyse the subcellular localization of DDB1 and DDB2. Whereas DDB1 was localized in both the nucleus and the cytoplasm, DDB2 was exclusively found in the nucleus. mRNA, were transfected individually into the MCF-7 cells at 100 nM for 24 h and the DDB2 protein level was analyzed by immunoblotting. All the DDB2 siRNA duplexes were able to knock down the expression of DDB2, albeit to varying degrees. This efficiency of these siRNA duplexes in the depletion of DDB2 expression was confirmed in COS-7 cells expressing stably Myc-His tagged DDB2. The siRNA duplex 3, which exhibited the highest efficiency to deplete cellular DDB2 protein, was introduced into the RNAi-ready pSIREN 8250835 vector for stable DDB2 suppression in the MCF-7 cells. Two stably DDB2 siRNA-transfected MCF-7 cell clones were isolated, where the DDB2 expression was strongly suppressed in contrast to that in the parent cells and in the stably transfected cells with a scrambled siRNA duplex sequence which were used as controls. The expression of DDB1 was not affected in the transfected cells, in contrast to that in the Wt MCF-7 cells. Growth curves were assessed by seeding cells in 24-well 24077179 tissue culture plates at 16104 cells/well and then cells were counted daily over a 4-day period. The two DDB2 deficient MCF-7 cell clones 2 and 3 showed an approximately similar growth ra
