The p63 gene encodes six protein isoforms. The transactivating isoforms have similar actions with p53, while the N-isoforms inhibit transcription activation by p53 and transactivating isoforms. p63 is expressed in stratified epithelia and in basal cells of the prostate and salivary glands. In mammary epithelium p63 has been shown to be expressed only in the myoepithelial layer. In the present study we investigated the immunohistochemical expression of p63, in benign and malignant breast lesions, and compared it with known myoepithelial cell markers. Our material consisted of 140 benign and 126 malignant breast lesions. We used the antibodies anti-p63, anti-alpha-smooth muscle actin, anti-S-100 protein and anti-cytokeratin 14. In all benign lesions, p63 immunoreactivity was noted in the myoepithelial cell layer surrounding the luminal epithelial cells. A less continuous peripheral rim of myoepithelial cells was also highlighted with p63-staining in all situ carcinomas. All invasive breast carcinomas were devoided of peripheral p63 staining. Interestingly, strong nuclear p63 immunoreactivity was noted in a small fraction (5-15%) of epithelial cells in all cases of papillomatosis, in 62.5% of in situ ductal papillary-type carcinomas and in 33.3% of invasive papillary carcinomas. Comparable staining was observed with S-100. The stromal cells were unreactive to p63. Our findings suggest that p63 is a sensitive and specific myoepithelial marker, and may be included in immunohistochemical panels aiming to identify myoepithelial cells in problematic breast lesions. Regarding papillary neoplasms, it is possible that tumor cells acquire and exhibit at least in part a myoepithelial differentiation program.