Abstract Purpose: Endocrine therapy is a well established therapy for estrogen receptor (ER) positive breast cancer. Two ER have been identified but currently only ERα is used in clinical practice. The role of ERβ in breast cancer is less well understood. Two human NM23 genes exist, NM23-H1 and NM23-H2. Several studies have shown an inverse relationship between overexpression of NM23-H1 in breast cancer and disease aggressiveness. NM23-H2, a metastasis suppressor protein, was recently identified as ERβ associated protein in vascular wall of atherosclerotic lesions, but the prognostic significance of this protein and its association with ERβ has not being studied in breast carcinoma. The purpose of this study was to investigate whether NM23-H2 is expressed in breast cancer and its potential clinical implications.Material and Methods: A cohort of 427 patients with diagnosis of primary invasive breast carcinoma between the period of 1994 to 1997 was used for this study. They all were patients diagnosed and treated at the Leeds Teaching Hospitals, UK. Immunohistochemistry was used primarily to identify the expression of NM23-H2 in this study. Regulation of NM23-H2 in vitro in MCF-7 cells was assessed by Western blotting. p<0.05 was considered statistically significant.Results: Patients ages were 27-92 years (median = 58 years, IQ range 47-69 years) with median follow up of 110 months. Tumour size varied from 1 and 130 mm (median=22.7mm, IQ range 14-26mm). Seventy eight percent were invasive ductal carcinoma NST and 11.5% were invasive lobular; 3.9% were special type (tubular, mucinous); and 6.6% were mixed type. Twenty-three percent of the tumours were grade 1, 43.3% were grade 2 and 33.7% were grade 3. Axillary metastases were present in 51% of the cases. NM23-H2 expression was variable between tumours and demonstrated only in the cytoplasm of breast carcinomas. The percentage of staining ranged from 0% to 100% (median 80%).NM23-H2 was strongly correlated with ERβ1 and ERα (p <0.001; correlation coefficient 0.252 and 0.176 respectively). No correlation was found between NM23-H2 with age, size, grade or lymph node status. NM23-H2 when expressed at levels higher than 30% was associated with improved overall survival (p= 0.030). In a multivariate Cox hazard analysis, NM23-H2 overexpression was a significant predictor of better survival independent of tumor grade, lymph node status, size or ERα. MCF-7 cells treated with 17β-estradiol (E2) expressed higher levels of NM23-H2(17.5kDa) compared with serum treated cells.Conclusion: This is the first time to our knowledge that the expression of NM23- H2 in breast cancer and its association with ERβ has been investigated. Our data provide evidence that immunohistochemical overexpression of NM23 H2 is associated with ERβ and ERα in breast cancer and with improved survival. NM23-H2 might be a good prognostic marker in breast cancer. Citation Information: Cancer Res 2009;69(24 Suppl):Abstract nr 4148.