Abstract Background: The widespread use of mammographic screening resulted in increased diagnosis of small (<2 cm; T1) tumors. Small tumors are associated with better prognosis, including a lower likelihood of developing metastasis, than larger tumors. Although this lower propensity to metastasize suggests that less aggressive treatments may be warranted in patients with T1 tumors, a subset of patients with small tumors (10-20%) will be diagnosed with lymph node metastasis. Methods: Frozen breast specimens were collected from women with T1 tumors and either negative (n=29) or positive (n=15) lymph node status. RNA was isolated from pure tumor cell populations after laser microdissection. Gene expression data was generated using HG U133A 2.0 arrays (Affymetrix). Differential expression was determined using Mann-Whitney testing using a P-value < 0.001 to define significance. Results for ESR1 were validated by immunohistochemistry. Results: Tumor characteristics did not differ significantly between groups in terms of age at diagnosis, grade, HER2 or PR status; however, tumors from patients with positive lymph nodes (47%) were significantly (P<0.05) more frequently ER negative compared to node negative (14%) patients. Gene expression analysis revealed 17 genes that were differentially expressed between node negative and node positive tumors: 6 with higher expression in node positive, including AURKA, and 11 with higher expression in node negative patients, including ESR1 and EPHX2. Of note, ESR1 was expressed at >4X higher levels in tumors without metastasis, in agreement with IHC findings. Conclusions: Small metastatic tumors differ in gene expression from those without metastasis. EPHX2 has been implicated as a metastasis suppressor while AURKA has been implicated as a metastasis promoter. These results suggest that small tumors have different propensities to metastasize and the genetic signature may serve as a new molecular tool to discriminate metastatic and non-metastatic small tumors, allowing appropriate treatment and risk assessment to be performed. Citation Information: Cancer Res 2010;70(24 Suppl):Abstract nr P4-06-09.