Phyllodes tumor of the breast is a rare fibroepithelial lesion characterized by a propensity for local recurrence and distant metastasis. Its histologic classification, based on morphology, mitotic index and tumor margin, includes malignant, borderline, and benign. These tumors show similar cellular morphology, which may contribute to difficulty in classifying them histologically and in prediction of their clinical behavior. Thus, the identification of new biomarkers detectable also by in situ methods has become indispensable. Paralogous HOX13 genes (HOX A13, HOX B13, HOX C13 and HOX D13) play a relevant role in tumor development and progression in particular in breast cancer. In this study we analyzed the immunohistochemical expression of paralogous HOX13 homeoproteins on a phyllodes tumor case series to validate their usefulness in histologic classification.