Keratin 21 is a

type I cytokeratin Type I keratins (or Type I cytokeratins) are cytokeratins that constitute the Type I intermediate filaments (IFs) of the intracytoplasmatic cytoskeleton, which is present in all mammalian epithelial cells. Most of the type I keratins consist of acid ...

which expresses immunologically specific

fusion protein Fusion proteins or chimeric (kī-ˈmir-ik) proteins (literally, made of parts from different sources) are proteins created through the joining of two or more genes that originally coded for separate proteins. Translation of this ''fusion gene'' r ...

. It is not found in humans, but only in '' Rattus norvegicus''. It is first detectable after 18-19 days of gestation.

References

Keratins Mammalian proteins {{Protein-stub