Editing Keratin (section)

== Examples of occurrence ==
[[File:Male impala profile.jpg|thumb|The [[Horn (anatomy)|horns]] of the [[impala]] are made of keratin covering a core of [[bone]].]]
[[Alpha-keratin]]s (α-keratins) are found in all vertebrates. They form the [[hair]] (including [[wool]]), the [[stratum corneum|outer layer of skin]], [[horn (anatomy)|horns]], [[nail (anatomy)|nails]], [[claws]] and [[Hoof|hooves]] of mammals, and the slime threads of [[hagfish]].<ref name="Wang 2016"/> The [[baleen]] plates of filter-feeding [[whale]]s are also made of keratin. '''Keratin filaments''' are abundant in [[keratinocyte]]s in the hornified layer of the [[epidermis]]; these are proteins which have undergone [[keratinization]]. They are also present in epithelial cells in general. For example, mouse thymic epithelial cells react with [[Antibody|antibodies]] for keratin 5, keratin 8, and keratin 14. These antibodies are used as [[Fluorescent tag|fluorescent markers]] to distinguish subsets of mouse [[Thymus|thymic]] epithelial cells in genetic studies of the [[thymus]].

The harder [[beta-keratin]]s (β-keratins) are found only in the [[sauropsid]]s, i.e., all living [[reptile]]s and [[bird]]s. They are found in the nails, [[scale (zoology)|scales]], and claws of reptiles, in some reptile [[Exoskeleton|shells]] ([[Testudines]]), and in the [[feather]]s, [[beak]]s, and claws of birds.<ref>{{Cite book | vauthors = Hickman CP, Roberts LS, Larson AL | title = Integrated principles of zoology | location = Dubuque, IA | pages = [https://archive.org/details/isbn_9780072930283/page/538 538] | year = 2003 | publisher = McGraw-Hill | isbn = 978-0-07-243940-3 | url-access = registration | url = https://archive.org/details/isbn_9780072930283/page/538 }}</ref> These keratins are formed primarily in [[beta sheet]]s. However, beta sheets are also found in α-keratins.<ref>{{cite journal | vauthors = Kreplak L, Doucet J, Dumas P, Briki F | title = New Aspects of the α-Helix to β-Sheet Transition in Stretched Hard α-Keratin Fibers | journal = Biophysical Journal | volume = 87 | issue = 1 | pages = 640–647 | date = July 2004 | pmid = 15240497 | pmc = 1304386 | doi = 10.1529/biophysj.103.036749 | bibcode = 2004BpJ....87..640K }}<!--|access-date=15 April 2016--></ref><!-- what exactly does this reference refer to? Not the distribution of a- and b-keratins it seems -->
Recent scholarship has shown that sauropsid β-keratins are fundamentally different from α-keratins at a genetic and structural level. The new term ''corneous beta protein'' (CBP) has been proposed to avoid confusion with α-keratins.<ref>{{cite journal | vauthors = Alibardi L | title = Sauropsids Cornification is Based on Corneous Beta-Proteins, a Special Type of Keratin-Associated Corneous Proteins of the Epidermis | journal = Journal of Experimental Zoology. Part B, Molecular and Developmental Evolution | volume = 326 | issue = 6 | pages = 338–351 | date = September 2016 | pmid = 27506161 | doi = 10.1002/jez.b.22689 | bibcode = 2016JEZB..326..338A }}</ref>

Keratins (also described as [[cytokeratins]]) are [[polymer]]s of type I and type II [[intermediate filaments]] that have been found only in [[chordate]]s ([[vertebrate]]s, [[Lancelet|amphioxi]], [[Tunicate|urochordates]]). [[Nematode]]s and many other non-chordate animals seem to have only type VI [[intermediate filaments]], [[lamins|fibers that structure the nucleus]].