The keratin gene family is a large group of intermediate filament and structural proteins that provide the structural components of hair, nails, horns, scales, claws and similar types of hard tissues. Keratins are also important for intracellular stability in epithelial tissues and different keratins often display organ- or tissue specific expression patterns. Due to these specific expression patterns, keratins are often used as diagnostic biomarkers. Keratin 19 is a type I keratin and expressed in simple epithelia including glandular cell types present in the GI-tract, female tissues, male tissues, and respiratory epithelium. Clinically, keratin 19 is used together with keratin 18 to distinguish hepatocellular cancer from cholangiocellular carcinoma (both keratins are expressed in bile ducts, but keratin 18 is expressed in hepatocytes whereas keratin 19 is not). Keratin 19 is a type I keratin. The type I cytokeratins consist of acidic proteins which are arranged in pairs of heterotypic keratin chains. Unlike its related family members, this smallest known acidic cytokeratin is not paired with a basic cytokeratin in epithelial cells. It is specifically found in the periderm, the transiently superficial layer that envelops the developing epidermis. The type I cytokeratins are clustered in a region of chromosome 17q12-q21.