Cysteine / Cys / C (EnzDes coloring)
Cysteine (CYS or C) is a hydrophobic amino acid. Its shape in Foldit is identical serine. The CPK or EnzDes coloring options (under view options) can be used to distinguish cysteine from serine.
Cysteine can bond with another cysteine, forming a disulfide bridge. Disulfide bridges help to stabilize proteins, especially those secreted from cells. Some puzzles in Foldit include a disulfide count condition that adds a bonus for each disulfide bridge formed.
A disulfide bridge removes one hydrogen from the sulfur of both cysteines, changing the atom count. Cysteine is the only amino acid where the atom count can change in mid-chain. Other amino acids have different atom counts only at the N terminal and C terminal ends of the chain. The atom count for a mid-chain cysteine changes from 11 to 10 when a disulfide bridge forms.
In the Foldit Lua interface, the functions structure.GetAminoAcid and structure.SetAminoAcid use lowercase "c" for cysteine.
For more, see Cysteine on wikipedia.
Disufide bridge