The transcript of epithelial-stromal interaction 1 (breast), EPSTI1, was originally found to be upregulated in invasive breast carcinomas as compared to normal breast tissue (Genomics 79:703, 2002). In the present study we investigated the expression, regulation and possible function of the protein product, EPSTI1. By use of a monoclonal antibody raised against a NH2-terminal peptide sequence the expression of native EPSTI1 was first analyzed by immunohistochemistry on tissue sections. EPSTI1 was primarily localized to the nucleus of both carcinoma and stromal cells with the highest expression in epithelial cells in close contact with stromal cells. Furthermore, preliminary data indicate that the expression was highest in estrogen receptor negative tumors. By comparison, in normal breast EPSTI1 was expressed primarily in the acini. To address a possible function of the protein we used a human breast epithelial cancer cell line, MCF-7, transfected with pRev-Tet-Off and pRevTRE with a FLAG-EPSTI1 insert. When cultured in 3-dimensional assays, the EPSTI1 expressing cells gave rise to more compact structures than the non-expressing cells, which resulted in loosely arranged/hollow structures. Immunocytochemistry indicated that EPSTI1 expressing cells also show elevated levels of STAT1 and á2â1-integrin, both of which have been implicated in the innate immune response. To investigate if EPSTI1 belongs to the group of interferon-response genes as suggested by others (Genome Biology 8:R191, 2007), we performed immunocytochemistry on a human breast epithelial cell line exposed to interferon-a. EPSTI1 as well as STAT1 were induced. Specific downregulation of EPSTI1 message with siRNAs also led to lower expression of á2â1-integrin. In conclusion, we have confirmed the existence of a protein product of EPSTI1 that is highly expressed in breast carcinomas at the epithelial-stromal interface as compared to normal breast. Furthermore, EPSTI1 is induced in response to interferon and may be involved in tissue organization.