Keratin derives from Greek κερατίνη keratíni from κέρας keras (genitive κέρατος keratos) meaning "horn" originating from the Proto-Indo-European *ḱer- of the same meaning. It is composed of "horn like", i.e., kerato, to which the chemical suffix -in is appended. The Greek keras is used in many animal names, e.g. Rhinoceros, meaning "nose with a horn".
Keratin filaments are abundant in keratinocytes in the cornified layer of the epidermis; these are proteins which have undergone keratinization. In addition, keratin filaments are present in epithelial cells in general. For example, mouse thymic epithelial cells (TECs) are known to react with antibodies for keratin 5, keratin 8, and keratin 14. These antibodies are used as fluorescent markers to distinguish subsets of TECs in genetic studies of the thymus.
Keratins (also described as cytokeratins) are polymers of type I and type II intermediate filaments, which have only been found in the genomes of chordates (vertebrates, Amphioxus, urochordates). Nematodes and many other non-chordate animals seem to only have type VI intermediate filaments, lamins, which have a long rod domain (vs. a short rod domain for the keratins).
The human genome encodes 54 functional keratin genes which are located in two clusters on chromosomes 12 and 17. This suggests that they have originated from a series of gene duplications on these chromosomes.
The keratins include the following proteins of which KRT23, KRT24, KRT25, KRT26, KRT27, KRT28, KRT31, KRT32, KRT33A, KRT33B, KRT34, KRT35, KRT36, KRT37, KRT38, KRT39, KRT40, KRT71, KRT72, KRT73, KRT74, KRT75, KRT76, KRT77, KRT78, KRT79, KRT8, KRT80, KRT81, KRT82, KRT83, KRT84, KRT85 and KRT86 have been used to describe keratins past 20.
The first sequences of keratins were determined by Hanukoglu and Fuchs. These sequences revealed that there are two distinct but homologous keratin families which were named as Type I keratin and Type II keratins. By analysis of the primary structures of these keratins and other intermediate filament proteins, Hanukoglu and Fuchs suggested a model that keratins and intermediate filament proteins contain a central ~310 residue domain with four segments in α-helical conformation that are separated by three short linker segments predicted to be in beta-turn conformation. This model has been confirmed by the determination of the crystal structure of a helical domain of keratins.