Sulfur-containing amino acid. HS-CH2-(Amino Acid Group). Most common residue involved in the formation of disulfide bonds when oxidized (only occurs on the exterior of cells, as the cellular environment is a reducing one). Common in hair, structural proteins and secreted amino acids.

Formula: C3H7NO2S
Molecular Weight: 121.16
Three-letter code: Cys
One-letter code: C