Cysteine
From FoldIt Wiki
Foldit's version of Cysteine
Cysteine (C) is a hydrophobic Amino acid. Its shape on Foldit is identical to that of Serine; however, Serine is hydrophilic and can be distinguished by its color.
Cysteine can form a covalent bond with another Cysteine in a different part of the molecule (or even to a Cysteine in a different subunit of the same protein). This bond is termed a Disulfide Bridge; these bridges help to stabilize proteins, especially those secreted from cells. The presence of a pair of Cysteines in a protein indicates the possibility (but not the requirement) of a Disulfide Bridge.
More information can be found at http://en.wikipedia.org/wiki/Cysteine
