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Background: Here we evaluate the expression and prognostic value of lysozyme, a milk protein that is also synthesized by a significant percentage of breast carcinomas, in women with breast cancer., Methods: Lysozyme expression was examined by immunohistochemical methods in a series of 177 breast cancer tissue sections. Staining was quantified by using the HSCORE system, which considers both the intensity and the percentage of cells staining at each intensity. The prognostic value of lysozyme was retrospectively evaluated by multivariate analysis that took into account conventional prognostic factors., Results: A total of 126 of 177 carcinomas (69.4%) stained positive for this protein, but there were clear differences among them with regard to the intensity and percentage of stained cells. Lysozyme values were higher in well-differentiated and moderately differentiated tumors than in poorly differentiated tumors (P < .05). Similarly, lysozyme levels were higher in small and node-negative tumors than in large and node-positive tumors (P < .05). Moreover, results indicated that low lysozyme content predicted shorter relapse-free survival and overall survival (P < .005). Separate Cox multivariate analysis in subgroups of patients as defined by node status showed that lysozyme expression was an independent prognostic factor able to predict both relapse-free survival and overall survival in node-negative patients (P < .05)., Conclusions: Tumoral expression of lysozyme is associated with lesions of favorable evolution in breast cancer. This milk protein may be a new prognostic factor in patients with breast cancer.