<s>[INST] context: Cytokeratins are proteins of keratin-containing intermediate filaments found in the intracytoplasmic cytoskeleton of epithelial tissue. The term "cytokeratin" began to be used in the late 1970s (for example, see "Intermediate-sized filaments of human endothelial cells" by Franke, Schmid, Osborn and Weber when the protein subunits of keratin intermediate filaments inside cells were first being identified and characterized. In 2006 a new systematic nomenclature for keratins was created and now the proteins previously called "cytokeratins" are simply called keratins. Cytokeratin Sites Cytokeratin 4 Non-keratinized squamous epithelium, including cornea and transitional epithelium Cytokeratin 7 A subgroup of glandular epithelia and their tumors Transitional epithelium and transitional carcinoma Cytokeratin 8 Glandular epithelia of the digestive, respiratory and urogenital tracts, both endocrine and exocrine cells, as well as mesothelial cells Adenocarcinomas originating from those above Cytokeratin 10 Keratinized stratified epithelium Differentiated areas of highly differentiated squamous cell carcinomas Cytokeratin 13 Non-keratinized squamous epithelia, except cornea Cytokeratin 14 Basal layer of stratified and combined epithelia Cytokeratin 18 Glandular epithelia of the digestive, respiratory, and urogenital tracts, both endocrine and exocrine cells, as well as mesothelial cells Adenocarcinomas originating from those above Cytokeratin 19 Glandular-type epithelia Does not react with hepatocytes and hepatocellular carcinoma[ Cytokeratin 20 Glandular-type epithelia. Signet ring/round clear cells [ GI stromal tumor (Krukenberg)[ [/INST] question: A 56 year old Chronic smoker, mass in bronchus resected. Most useful immunohistochemical marker to make a proper diagnosis would be | options: A: Cytokeratin B: Vimentin C: Epithelial membrane cadherin D: Leucocyte common antigen | answer: A </s>