Metastasis remains the primary cause of death in women with breast cancer. While it is well established that tumor cells are heterogeneous and minority populations within the tumor have tumor-initiating and metastatic capabilities, the identity of metastasis initiating cells (MICs) in human breast cancer remains controversial. Until we can identify these cells and study the molecular pathways regulating them, the development of new therapeutic strategies will be hindered. The long-term goal of this proposal is to understand the intrinsic and extrinsic molecular signaling regulating MICs. Recently, using single-cell RNA sequencing data from breast cancer patient samples and sophisticated computational approaches we identified an immature population of cells in breast cancer that express hematopoietic stem cell transcriptional adaptor and the T-cell oncogene, LMO2. The objective of this proposal is to determine the molecular mechanism of LMO2 in promoting metastasis. We hypothesize that LMO2+ cells are a population of MICs in breast cancer that are activated in response to inflammation and LMO2 is a key adaptor that regulates this process. Our hypothesis is based on our preliminary results that demonstrate that Lmo2+ cells are metastatic, predict poor distant recurrence-free survival in patients, LMO2 knockdown reduces metastasis in multiple human tumor models, and LMO2 is required for STAT3 activation in response to IL6 and TNFα. The rationale underlying this proposal is that αidentifying the signaling of LMO2 in metastasis will elucidate targets in MICs that are open to therapeutic intervention. Guided by strong preliminary data, our proposal will 1. Determine whether LMO2 is required for inflammation-induced metastasis. 2. Determine the detailed molecular signaling regulated by LMO2-STAT3 axis. 3. Determine whether LMO2+ cells utilize vascular mimicry to metastasize. The proposed research is significant because it will enable the design of therapeutic strategies targeting MICs in breast cancer. Previous research studies on the identification of MICs have relied on established lineage markers. The proposed research is innovative because we are focusing on a population of LMO2+ MICs that were agnostically identified from single-cell RNA sequencing in breast cancer patient samples. The proposed research will substantially enhance our understanding of MICs and lay the foundation for novel strategies to treat metastatic breast cancer.