The disulfide bond, also called a disulfide bridge, is a post-translational protein modification with significant and wide-reaching implications for protein structure and function. As the name suggests, a disulfide bond involves covalently linked sulphur atoms in cysteine amino acid residues and can be involved in both intra- and/or extra-molecular interactions. Disulfide bond-containing proteins, which are formed in the non-reducing environment of the endoplasmic reticulum, are found in almost all cell types. The biosynthetic pathways leading to the formation of disulfide bonds in mammalian cell systems are quite complex: Up to 40 proteins have been implicated in these processes.