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.
For more information, see Cysteine on Wikipedia.