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Pheomelanin production is highly dependent on [[cysteine]] availability, which is transported into the melanosome, reacting with dopaquinone to form cys-dopa. Cys-dopa then undergoes several transformations before forming pheomelanin.<ref name="Alaluf Heath Carter Atkins 2001 pp. 337–3472"/> In chemical terms, pheomelanins differ from eumelanins in that the oligomer structure incorporates [[benzothiazine]] and [[benzothiazole]] units that are produced,<ref>{{Cite journal |vauthors=Greco G, Panzella L, Verotta L, d'Ischia M, Napolitano A |date=April 2011 |title=Uncovering the structure of human red hair pheomelanin: benzothiazolylthiazinodihydroisoquinolines as key building blocks |journal=Journal of Natural Products |volume=74 |issue=4 |pages=675–82 |doi=10.1021/np100740n |pmid=21341762}}</ref> instead of DHI and [[DHICA]], when the amino acid [[L-cysteine]] is present.
Pheomelanins, unlike euemanins, are rare in lower organisms<ref name=":0" /> with claims they are an "evolutionary innovation in the tetrapod lineage"<ref name="eumelanin" /> but recent research finds them also in some fish.<ref name="Xuetal" />
=== Neuromelanin ===
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== Humans ==
[[File:Albinistic girl papua new guinea.jpg|thumb|[[Albinism in humans|Albinism]] occurs when melanocytes produce little melanin. This albino girl is from [[Papua New Guinea]].]]
In humans, melanin is the primary determinant of [[human skin color|skin color]]. It is also found in hair, the pigmented tissue underlying the [[iris (anatomy)|iris]] of the eye, and the [[stria vascularis of cochlear duct|stria vascularis]] of the [[inner ear]]. In the brain, tissues with melanin include the [[adrenal medulla|medulla]] and pigment-bearing neurons within areas of the [[brainstem]], such as the [[locus coeruleus]]. It also occurs in the [[zona reticularis]] of the [[adrenal gland]].<ref name=":0">{{Cite journal |last=Solano |first=F. |year=2014 |title=Melanins: Skin Pigments and Much More—Types, Structural Models, Biological Functions, and Formation Routes |journal=New Journal of Science |volume=2014 |pages=1–28 |doi=10.1155/2014/498276 |doi-access=free}}</ref>
The melanin in the skin is produced by [[melanocyte]]s, which are found in the [[Stratum germinativum|basal layer]] of the [[epidermis (skin)|epidermis]]. Although, in general, human beings possess a similar concentration of melanocytes in their skin, the melanocytes in some individuals and ethnic groups produce variable amounts of melanin. The ratio of eumelanin (74%) and pheomelanin (26%) in the epidermis is constant regardless of the degree of pigmentation.<ref name="i034">{{Cite journal |last1=Del Bino |first1=Sandra |last2=Ito |first2=Shosuke |last3=Sok |first3=Juliette |last4=Wakamatsu |first4=Kazumasa |date=2022 |title=5,6-Dihydroxyindole eumelanin content in human skin with varying degrees of constitutive pigmentation |journal=Pigment Cell & Melanoma Research |volume=35 |issue=6 |pages=622–626 |doi=10.1111/pcmr.13062 |issn=1755-1471 |pmc=9804219 |pmid=35933709}}</ref> Some humans have very little or no melanin synthesis in their bodies, a condition known as [[Albinism in humans|albinism]].<ref>{{Cite journal |last1=Cichorek |first1=Mirosława |last2=Wachulska |first2=Małgorzata |last3=Stasiewicz |first3=Aneta |last4=Tymińska |first4=Agata |date=20 February 2013 |title=Skin melanocytes: biology and development |journal=Advances in Dermatology and Allergology |volume=30 |issue=1 |pages=30–41 |doi=10.5114/pdia.2013.33376 |pmc=3834696 |pmid=24278043}}</ref>
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