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[sis-tee-een, -in] /ˈsɪs tiˌin, -ɪn/
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. Symbol: C.
Abbreviation: Cys;
Origin of cysteine
First recorded in 1880-85; alteration of cystine
Related forms
cysteinic, adjective Unabridged
Based on the Random House Dictionary, © Random House, Inc. 2018.
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British Dictionary definitions for cysteine


/ˈsɪstɪˌiːn; -ɪn/
a sulphur-containing amino acid, present in proteins, that oxidizes on exposure to air to form cystine. Formula: HSCH2CH(NH2)COOH
Derived Forms
cysteinic, adjective
Word Origin
C19: variant of cystine
Collins English Dictionary - Complete & Unabridged 2012 Digital Edition
© William Collins Sons & Co. Ltd. 1979, 1986 © HarperCollins
Publishers 1998, 2000, 2003, 2005, 2006, 2007, 2009, 2012
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cysteine in Medicine

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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cysteine in Science
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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