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It is one of the 20 standard amino acids the proteins in our body are made of. It isn’t an essential amino acid because it can be derived from serine or methionine, since this has been transformed into cystathionine.
Two cysteine molecules form cystine, an amino acid that is essential for keratinization and can be found in large quantities on the outer layer of hair. 
A diet lacking or low in cystine seems to determine a decreased hair production.

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