Mouse PEA3 is the founding member of the PEA3 subfamily of ETS transcription factors that includes ERM and ER81. Numerous studies implicate PEA3 subfamily members in a diversity of human cancers, especially breast cancer. Dominant-negative PEA3 (L1NPEA3En) effectively represses activated transcription by all three PEA3 subfamily members. When expressed under control of the MMTV promoter, L1NPEA3En significantly delays the appearance of mammary tumors and reduces their number and size in mouse models of HER2 mediated breast cancer. In addition, L1NPEA3En is not expressed in the mammary tumors that do develop in these mice. These findings strongly suggest a required role for PEA3 subfamily members or other ETS proteins with similar DNA binding specificity in HER2-mediated oncogenesis. The primary objective of this research was to identify the PEA3 subfamily target genes that could play a role in the initiation and progression of tumors, specifically in the breast. To achieve this, a recombinant adenovirus carrying L1NPEA3En was constructed to express L1NPEA3En in three human mammary tumor cell lines: MDA-MB-468, BT-549 and MDA-MB-361. Gene expression analysis using Affymetrix® GeneChip® technology identified a common set of 39 downregulated and 2 upregulated genes in cells expressing L1NPEA3En compared to control cells in all three tumor cell lines. Differentially expressed genes included some that have been shown to play key roles in tumorigenesis such as activating transcriptionfactor 3, heat shock 70kD protein lA and interleukin-8. In addition one colon carcinoma cell line, SW620, was used for gene expression analysis and 7 genes identified in the mammary tumor cell lines were also identified in the colon carcinoma cell line. The results suggest a role for PEA3 subfamily genes in a multiple human cancers mediated through a small subset of common target genes. The genes identified as being differentially expressed by ~NPEA3En hold potential value not only as targets for therapeautic drug discovery, but also as diagnostic or prognostic markers for human cancers, specifically breast cancer. / Thesis / Master of Biological Science (MBioSci)