This antibody labels the high molecular weight (HMW) cytokeratin (CK5) protein in both normal and neoplastic cells. Cytoplasmic staining is observed in stratified squamous epithelia, and in basal cells of complex epithelia. Simple epithelia and nonepithelial cells are negative. It is a useful tool for classifying squamous cell carcinoma, distinguishing malignant mesothelioma from lung adenocarcinoma, and determining breast and prostate malignancies when used in a panel of antibodies.