[ sis-tee-een, -in ]
/ ˈsɪs tiˌin, -ɪn /

noun Biochemistry.

a crystalline amino acid, C3H7O2NS, a component of nearly all proteins, obtained by the reduction of cystine. Abbreviation: Cys; Symbol: C

Origin of cysteine

First recorded in 1880–85; alteration of cystine

Related forms

cys·te·in·ic, adjective Unabridged Based on the Random House Unabridged Dictionary, © Random House, Inc. 2019

Examples from the Web for cysteine

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: HSCH 2 CH(NH 2)COOH

Derived Forms

cysteinic, adjective

Word Origin for cysteine

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

Medicine definitions for cysteine


[ sĭstē-ēn′, -ĭn, sĭ-stēĭn ]


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.

Science definitions for cysteine


[ sĭstə-ēn′ ]

A nonessential amino acid. Chemical formula: C3H7NO2S. See more at amino acid.

The American Heritage® Science Dictionary Copyright © 2011. Published by Houghton Mifflin Harcourt Publishing Company. All rights reserved.