Cytokeratins are a family of intermediate filament proteins that assemble into filaments through forming heterodimers of one type I Cytokeratins (Cytokeratins 9 to 23) and one type II Cytokeratins (keratins 1 to 8). The cytokeratin proteins play a critical role in differentiation, as well as tissue specialization and function, to maintain the overall structural integrity of epithelial cells. Cytokeratins are also useful markers in identifying the origin of metastatic tumors. Cytokeratin 16 is expressed in benign stratified squamous epithelium and squamous cell carcinoma of the head and neck, as well as luminal cells of mammary gland and sweat ducts. It is absent in non-invasive breast carcinomas and normal breast tissue.