Ets proteins are a family of transcription factors that share an 85 amino acid conserved DNA binding domain, the ETS domain. The 27 known human Ets transcription factors control multiple biological processes, including cellular proliferation, differentiation, apoptosis, angiogenesis, transformation, and invasion. Overexpression of some Ets genes has been linked to numerous malignancies, including breast cancers. The prostate derived Ets transcription factor (PDEF) is reported to be a breast and prostate tumor-associated Ets factor. To understand the roles of PDEF in breast cancers, we transiently overexpressed PDEF in MDA-MB-231 human breast cancer cells by adenoviral-mediated gene delivery. PDEF binding protein complexes were isolated by immunoprecipitation and PDEF-interacting proteins were analyzed by LC-MS/MS. After subtracting the proteins binding nonspecifically to antibody-bead complexes, we identified 286 proteins in the PDEF-associated protein complex. By comparison to published protein-protein interactions, we selected 121 proteins for further analysis. PDEF interactors distribute not only in the nucleus, but also in the cytoplasm, as well as other subcellular compartments. Our data reveals that PDEF interacts with a variety of proteins involved in cell cycle, DNA repair, cytoskeleton organization, mRNA processing, tRNA biosynthesis, protein folding, and cell signaling. Furthermore, the EGFR1- (Erbb1) and Erbb2- (HER2) related proteins erbin, an ERBB2 interacting protein, catenin delta-1 (which interacts with Erbin), and EGFR (a HER2-homology receptor) were associated with PDEF. These findings indicate that PDEF may be regulated by Erbb2 or EGFR-activated signaling pathways in breast cancer cells. Further analysis of these proteins will identify the roles of PDEF-interacting proteins in breast tumorigenesis.