Cytokeratin 5 is a 58-kDa protein found in many non-keratinizing, stratified squamous epithelia such as tongue mucosa, basal epithelia hair follicles, trachea, as well as basal cells in prostate and mammary glands. CK5 is also expressed in most epithelial and biphasic mesotheliomas. p63 is detected in prostatic basal cells in normal prostate; however, it is negative in malignant tumors of the prostate gland. Thus, p63 may be useful to aid in the differentiation of benign and malignant tumors of prostate gland. It has been reported that p63 may be useful as a negative marker for malignant mesotheliomas. p63 also stains basal cells in mammary glands.