Public reporting burden for this collection of information is estimated to average 1 hour per response, including the time for reviewing instructions, searching existing data sources, gathering and maintaining the data needed and completing and reviewing this collection of information. Send comments regarding this burden estimate or any other aspect of this collection of information, including suggestions for reducing this burden to Washington Headquarters Services, Directorate for Information Operations and Reports, 1215 Jefferson Davis Highway, Suite 1204. Arlington, VA 22202-4302, and PERFORMING ORGANIZATION NAME(S) AND ADDRESS(ES)Long Island Jewish Medical Center New Hyde Park, New York 11040 E-Mail: shi@lij.edu SPONSORING / MONITORING AGENCY NAME(S) AND ADDRESS(ES)U AbstractWe have previously identified and characterized a novel tumor growth inhibitor and a fatty acid binding protein in human mammary gland and named it as Mammary derived growth inhibitor Related Gene MRG. MRG has tumor-suppressing activities; it inhibits breast cancer cell growth in vitro and tumor growth in vivo. Here, the effects of MRG on mammary gland differentiation and its interaction with o>3 polyunsaturated fatty acids (o>3 PUFA) on growth inhibition were investigated. MRG protein expression was associated with human mammary gland differentiation with the highest expression observed in the differentiated alveolar mammary epithelial cells from the lactating gland. Overexpression of MRG in human breast cancer cells induced differentiation with changes in cellular morphology and a significant increase in the production of lipid droplets. Treatment of mouse mammary gland in organ culture with MRG protein resulted in a differentiated morphology and stimulation of ß-casein expression. Treatment of human breast cancer cells with u>3 PUFA DHA resulted in a differential growth inhibition proportional to their MRG expression. MRG transfected cells or MRG protein treated cells were much more sensitive to DHA-induced growth inhibition compared with MRG negative or control non-treated cells. Our results suggest that MRG is a candidate mediator of the differentiating effect of pregnancy on breast epithelial cells and may play a major role in u>3 PUFA-mediated tumor suppression.