This protein is a member of the keratin family. 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 expressed in the periderm, the transiently superficial layer that envelopes the developing epidermis. Keratin 19 is not expressed in hepatocytes, therefore, antibody to keratin 19 is useful in the identification of liver metastasis. The degree of keratin 19 positivity in breast cancer distinguishes malignant from benign tumours. Keratin 19 is often co-expressed with keratin 7.