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Abstract. CCN1 plays diverse roles in cellular proliferation, survival, migration and angiogenesis. We determined the relationship between CCN1 protein expression and clinical factors that are important for the classification of breast cancer. CCN1 contains four functional domains; the contribution of each of the structural domains to the biological properties of CCN1 in breast cancer was investigated. We performed immunohistochemistry for CCN1 on a breast cancer tissue array, and conducted a detailed statistical analysis on the relationship between CCN1 protein expression and clinical factors that are important for the classification of breast cancer. The structure-function relationship was examined using four mutant constructs in which one of the modules (DM1-DM4) had been deleted. MCF-7 breast cancer cells were stably transfected with these constructs and their biological activity was tested in comparison to full-length CCN1. Staining of CCN1 in tumors was positively correlated with AJCC disease stage. A strong association also was found between lymph node involvement and high CCN1 expression in patients with invasive breast cancer; there was a significant increase in the breast cancer expression of CCN1 in patients with positive lymph nodes (P=0.004), and the levels of CCN1 correlated with the number of positive lymph nodes (P=0.0006). Deletion of module 4 rendered CCN1 unable to either bind heparin or associate with the extracellular matrix. Furthermore, MCF-7/DM4 cells demonstrated reduced cell spreading, migration and proliferation, indicating that module 4 of the protein is important for its ability to promote these activities. These findings indicate that CCN1 is involved throughout the clinical progression of breast cancer to an invasive phenotype. The multimodular structure of CCN1 enables it to fulfill multiple functions that may contribute to the different stages of cancer development, raising the prospect that specific regions of CCN1 could be targeted for therapeutic benefit to inhibit particular aspects of malignancy in breast cancer.