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I work as a structural biochemist and I contest the statement that cysteine is an uncountable noun. This is true for those thinking on the macroscale about the amino acid as a chemical substance, but is untrue for those working with specific proteins. Specific proteins contain a countable number of each amino acid, one of which is cysteine. It is very common to hear a statement like "protein disulfide isomerase contains two cysteines." Just google the phrase "contains two cysteines" in quotes. You'll get many examples.

That may be so, but it is still dodgy English - the correct terminology is "cysteine residues". --Williamsayers79 19:31, 9 October 2008 (UTC)