Cytokeratins are intermediate filamentous proteins that are expressed in epithelial cells. They vary in size from approx. 40 to 68 kDa. The family comprises 20 polypeptides, subdivided into type I (CK9-CK20, acidic, smaller polypeptides) and type II (CK1-CK8, basic or neutral, larger polypeptides). Cytokeratins have a common basic structure with four conserved domains and form intermediate filaments of approx. 10 nm in diameter through dimerization and tetramers. They are expressed in pairs of type I and II, e.g. CK10 with CK1 .

Epithelial cells use different cytokeratin subtypes for classification and diagnostics. Cytokeratins are important for cell stability, integrity and signaling pathways such as wound healing and apoptosis. They serve as immunohistochemical tumor markers; pan-cytokeratin antibodies recognize several subtypes.

Mutations in cytokeratins cause diseases such as epidermolysis bullosa simplex (EBS), an autosomal dominant disease caused by mutations in CK5 or CK14 . Other diseases include bullous ichthyosis and epidermolytic palmoplantar keratoderma (EPPK) with skin changes.