cys·​te·​ine ˈsi-stə-ˌēn How to pronounce cysteine (audio)
: a crystalline sulfur-containing amino acid C3H7NO2S readily oxidizable to cystine

Example Sentences

Recent Examples on the Web The chemical used to treated acetaminophen poisoning, N-acetyl cysteine, is meant to protect the liver from the damage of a single overdose, and is unlikely to help patients with staggered overdoses, the researchers write. Douglas Main, Discover Magazine, 24 Nov. 2011 However, the star ingredient in PhenQ is α-Lacys Reset, which combines alpha-lipoic acid (a natural antioxidant), L-cysteine (an amino acid), and magnesium to burn body fat and boost your metabolic rate. Dallas News, 7 Aug. 2022 Two important sulfur-containing amino acids, cysteine and methionine, degrade during the fermentation process and form various sulfides that are responsible for those sulfurous, meaty, and onion odors [3]. Catherine Hu, Discover Magazine, 8 Dec. 2015 Whey protein also contains high amounts of leucine, the most important amino acid in muscle growth, and cysteine, which aids in muscular development and metabolism. Amber Smith, Discover Magazine, 16 Dec. 2022 The new treatment is an inexpensive, over-the-counter supplement called N-acetyl cysteine, or NAC, that is widely available. Cezary Podkul, WSJ, 28 Dec. 2020 The researchers received funding from Catapult Cat Oy, which sells the L-cysteine supplements. Andrew Stuttaford, National Review, 20 Aug. 2020 The subjects were then asked to swallow placebo or L-cysteine tablets containing vitamin supplements., 19 Aug. 2020 The researchers received funding from Catapult Cat Oy, which sells the L-cysteine supplements. Andrew Stuttaford, National Review, 20 Aug. 2020 See More

These example sentences are selected automatically from various online news sources to reflect current usage of the word 'cysteine.' Views expressed in the examples do not represent the opinion of Merriam-Webster or its editors. Send us feedback.

Word History


International Scientific Vocabulary, from cystine + -ein

First Known Use

1884, in the meaning defined above

Time Traveler
The first known use of cysteine was in 1884

Dictionary Entries Near cysteine

Cite this Entry

“Cysteine.” Dictionary, Merriam-Webster, Accessed 6 Feb. 2023.

Medical Definition


cys·​teine ˈsis-tə-ˌēn How to pronounce cysteine (audio)
: a sulfur-containing amino acid C3H7NO2S occurring in many proteins and glutathione and readily oxidizable to cystine
abbreviation Cys

More from Merriam-Webster on cysteine

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