Keratins are cytoplasmic intermediate filament proteins expressed by epithelial cells. Cytokeratins, 5 & 6, are type II keratins that are in combination useful in differentiating mesothelioma (positive) from lung carcinoma (negative) or metastic carcinoma in the pleura (negative). It detects undifferentiated large cell carcinoma, squamous carcinoma, spindle cell squamous cell carcinoma of the skin, malignancies of prostate and breast and it helps to discriminate between ductal hyperplasia of the breast (strong staining) and solid papillary DCIS (negative).