cysteine

noun

cys·​te·​ine ˈsi-stə-ˌēn 

: a crystalline sulfur-containing amino acid C₃H₇NO₂S readily oxidizable to cystine

Examples of cysteine in a Sentence

Recent Examples on the Web

Tofu is a complete protein with all nine essential amino acids, while cottage cheese is an incomplete protein, lacking cysteine. — Mark Gurarie, Verywell Health, 15 Sep. 2026

Such a meal should include a good source of protein, such as eggs, in order to stock up on or refuel depleted amino acids, including cysteine and methionine, which are building blocks for protective antioxidants. — Lauren J. Young, Scientific American, 17 Mar. 2026

Each video is great, but her reaction to tasting cysteine is memorable. — Brittany Trang, STAT, 4 Feb. 2026

These forms differ at two positions in the protein, called 112 and 158, where the building blocks cysteine or arginine can appear in different combinations. — William A. Haseltine, Forbes.com, 27 Jan. 2026

Word History

Etymology

International Scientific Vocabulary, from cystine + -ein

First Known Use

1884, in the meaning defined above

Medical Definition

cysteine

noun

cys·​teine ˈsis-tə-ˌēn 

: a sulfur-containing amino acid C₃H₇NO₂S occurring in many proteins and glutathione and readily oxidizable to cystine

—abbreviation Cys