In normal tissues, CK5 is expressed in the basal cells of squamous epithelium and ductal epithelium, and part of the germinal cells of the squamous epithelium, myoepithelial cells and mesothelial cells. While it is almost not expressed in the vast majority of glandular epithelial cells. Therefore, it can be used for differential diagnosis of squamous cell carcinoma and adenocarcinoma, mesothelioma and adenocarcinoma. And it can also be used for differential diagnosis of mammary gland, salivary gland, benign and malignant tumor derived from prostate epithelial.