[sis-tee-een, -in]
noun Biochemistry.
a crystalline amino acid, C 3 H 7 O 2 NS, a component of nearly all proteins, obtained by the reduction of cystine. Abbreviation: Cys; Symbol: C

1880–85; alteration of cystine

cysteinic, adjective Unabridged
Based on the Random House Dictionary, © Random House, Inc. 2014.
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World English Dictionary
cysteine (ˈsɪstɪˌiːn, -ɪn)
a sulphur-containing amino acid, present in proteins, that oxidizes on exposure to air to form cystine. Formula: HSCH2CH(NH2)COOH
[C19: variant of cystine]

Collins English Dictionary - Complete & Unabridged 10th Edition
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American Heritage
Medical Dictionary

cysteine cys·te·ine (sĭs'tē-ēn', -ĭn, sĭ-stē'ĭn)
Abbr. Cys
An alpha-amino acid found in most proteins and especially abundant in keratin.

The American Heritage® Stedman's Medical Dictionary
Copyright © 2002, 2001, 1995 by Houghton Mifflin Company. Published by Houghton Mifflin Company.
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American Heritage
Science Dictionary
cysteine   (sĭs'tə-ēn')  Pronunciation Key 
A nonessential amino acid. Chemical formula: C3H7NO2S. See more at amino acid.
The American Heritage® Science Dictionary
Copyright © 2002. Published by Houghton Mifflin. All rights reserved.
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Example sentences
These break down the protein residues and release methyl mercaptan from the amino acids cysteine and methionine.
The cysteine thiol group is nucleophilic and easily oxidized.
Cysteine is an important source of sulfide in human metabolism.
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