Abstract Background: The breast tumor microenvironment is characterized by the release of endogenous reactive oxygen species (ROS), and has been suggested as being associated with disease aggressiveness. Normal cells have an antioxidant system that controls the balance between production and removal of oxygen radicals, thereby protecting against oxidative damage, such as the Glutathione peroxidases (GPXs) enzymes. Epigenetics and DNA methylation play important roles in several inflammatory disorders. We therefore, analyzed the promoter of GPX3 and found a dense CpG island closest to the transcription start-site. The aim of the present study is to 1) investigate GPX3 expression in breast carcinoma and normal breast tissues, 2) identify whether GPX3 is epigenetically regulated in breast carcinoma tissues versus normal tissue, and 3) compare the levels of expression of GPX3 in Inflammatory Breast Cancer (IBC) versus non-IBC tissues. Material and Methods: We enrolled 40 breast cancer patients with tumor mass range from 1.8-9cm (with median size 4.77 ± 3.9). Patients were sub-grouped as IBC (n = 20) or non-IBC (n = 20), healthy breast tissues from the same patients were used as control. Using real-time PCR and immunohistochemistry, we assessed the level of expression of GPX3 at mRNA and protein levels in breast cancer tissues versus control. To identify whether the CpG island of the GPX3 gene was epigenetically regulated, we analyzed the methylation profile of GPX3 in breast carcinoma versus normal tissues using DNA bisulfate treatment and methylation-specific PCR (MSP). Results: GPX3-mRNA expression was down regulated in breast cancer samples compared to control tissues. There was a significant decrease (P = 0.01) in the mRNA expression level of IBC compared to non-IBC carcinoma tissues. MSP showed that the GPX3 gene was hypermethylated in breast carcinoma tissues compared to control. Moreover, confirmatory semiquantitative immunohistochemical scoring revealed weak or negative immunostain of carcinoma tissues compared to healthy breast tissues. Conclusion: These preliminary data suggest that epigenetic inactivation of GPX3 is a frequent finding in inflammatory breast cancer. Silencing of GPX3 in IBC versus non-IBC carcinoma cells suggested that GPX3 may be critical in the development and progression of IBC. To our knowledge, this is first study to test the role of GPX3 in breast cancer. Citation Information: Cancer Res 2010;70(24 Suppl):Abstract nr P3-04-05.