Test Details: Immunohistochemical testing helps to identify specific antigens (proteins) within tissue sections utilizing an antigen-specific antibody. Cytokeratin 18 is a type I cytokeratin, an intermediate filament expressed in simple epithelial cells and found in the majority of non-squamous carcinomas including adenocarcinoma of unknown origin and neuroendocrine carcinoma in addition to hepatocellular and renal cell carcinoma.